Turkey's Extraterritorial Use of Force against Armed Non-State Actors

Abstract The use of force in foreign territories has been contained in the Constitution of the Republic of Turkey, with the authorisation of the Grand National Assembly of Turkey, in ‘cases deemed legitimate by international law’ and where required by international treaties to which Turkey is a party. Yet Turkey's extraterritorial use of force against armed non-state actors lead to the most important question of identifying the circumstances under which the Turkish authorities have long justified military intervention in foreign territories. This article aims to assess whether Turkey's use of force and alleged extraterritorial self-defence contravenes international law. In order to address how Turkey interprets the right to use armed force and the right of self-defence, and to bring clarity to the state's approach to international law on the use of force (jus ad bellum), the article explores Turkey's practice based on the assessment of the Turkish military intervention in Syria, in line with both bilateral security or defence treaties to which Turkey is a party and the use of force in self-defence. The aim is to determine whether Turkey's justifications are compatible with the jus ad bellum criteria.


Introduction
The use of armed force is generally prohibited under Article 2(4) of the Charter of the United Nations, which states: 1 All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations.
There are, however, numerous examples in practice in which states have justified military intervention in foreign territories, under bilateral treaties or self-defence, against imminent attacks by armed non-state actors, where the territorial state is unwilling or unable to take necessary measures. This determination requires an assessment of how states classify their military operations under the law of self-defence in international law. In doing so, the article sets out an actual example of a set of regulations that legitimise the use of force and deployment of armed forces by Turkey, which is addressed in greater detail below. Turkey has often been the victim of terrorist attacks launched by the Kurdistan Workers' Party (PKK), which has been categorised as a terrorist group by Turkey and many other states and international organisations, including the United States and European Union. 2 Turkey has occasionally responded to PKK attacks, coming both from south-eastern Turkey and the Qandil mountains in northern Iraq, since the 1980s. However, Turkey's military operations have been controversial regarding their classification as self-defence under the UN Charter and jus ad bellum. 3 Jus ad bellum is a set of criteria under which states construct the right to use armed force and is key to understanding the circumstances in which rights and obligations of states are acquired in the law governing the use of force. 4 This is, of course, distinct from jus in bello, which is known as a set of criteria or rules that ought to be followed during the conflict. 5 For the purpose of this article it is instructive to consider the conditions that are required for states to exercise the right to use armed force in foreign territories. 6 Having discussed Turkey's practice, this article explores the current debates in recent literature to shed light on the legality of the Turkish military intervention in northern Syria. That being said, military intervention in Syria is the primary focus of the article as it has generated much debate on whether the precepts of jus ad bellum have been satisfied. It thus raises a more fundamental critique of states' own assessment of a particular situation as armed aggression. In so doing, the article challenges Turkey's basic assumptions of the right to use force and military intervention in northern Syria, justified by the necessity of self-defence.
The article builds on critical questions of jus ad bellum as to whether the right to use armed force has been interpreted and applied adequately by the Turkish authorities in compliance with jus ad bellum. While the discussion reflects Turkey's approach to jus ad bellum, the article also answers the more challenging legal question of whether military intervention and the use of force against Kurdish and the so-called Islamic State fighters in Syria are compatible with the ad bellum criteria. If the legality does exist, an objective question relates to the implications of Turkey's military presence in Syria in the aftermath of the fall of Islamic State. Having delineated the scope of selfdefence in international law, the article examines the various aspects of Turkey's approaches to jus ad bellum to shed light on the ad bellum criteria that allow the use of armed force which does not impinge on the territorial integrity of a state. This article aims to argue that Turkey's extraterritorial selfdefence operations cannot be inferred from a reading of Article 51 of the UN Charter. While the article addresses the question of Turkey's compliance with jus ad bellum, it lies primarily in the analysis of Turkey's agreements with Syria as a basis for recourse to armed force.
The analysis proceeds as follows. Section 2 draws on preliminary considerations for the main analysis, where it considers the general prohibition of the use of force and the exceptions to this prohibition as the core components of jus ad bellum. Section 3 explores the Turkish military intervention in northern Syria asserted in line with the bilateral security treaties signed between International Law 921, 927; Robert Kolb, 'Origin of the Twin Terms Jus ad Bellum/Jus in Bello' (1997) 37 International Review of the Red Cross 553; Christopher Greenwood, 'The Relationship between Ius ad Bellum and Ius in Bello' (1983) 9 Review of International Studies 221, 230. 6 The right to use armed force is recognised both under customary international law and the UN Charter. It is also widely accepted that this right has its root in customary international law and has subsequently been codified by the UN Charter. Relatedly, it is also worth noting that the right of self-defence contained in Article 51 of the UN Charter has attained the status of jus cogens in international law and corresponds entirely with the inherent right of self-defence to be found in customary international law. Tom Ryus has argued, for example, that both the prohibition of the use of force contained in Article 2(4) and the right of self-defence contained in Article 51 of the UN Charter are jus cogens norms, which are closely associated with the application of a particular set of norms in customary international law: Tom Ruys Turkey and Syria as primary justifications for using armed force on Syrian territory. Section 4 looks at how Turkey relies on the applicability of jus ad bellum and self-defence in using force against armed non-state actors, and the measures taken by Turkey as a result of the unwillingness or inability of the Syrian government. This part of the research then examines self-defence against imminent attacks and the armed attack threshold for using force in self-defence, according to which Turkey has relied on the ad bellum criteria by considering certain actions as armed attacks.

Preliminary considerations
Jus ad bellum is the only branch of international law that governs the conditions under which states may resort to the use of armed force in general. It is governed by conventional (including Articles 2(4), 39-42 and 51 of the UN Charter) and customary international law. 7 To begin, the general prohibition of the use of force is regulated by Article 2(4) of the UN Charter, and the exceptions to this prohibition (use of force in self-defence and use of force with UN Security Council authorisation) 8 set out in Chapter VII of the UN Charter are the core components of jus ad bellum. The prohibition of the use of force is not only recognised as a customary norm, 9 but is generally considered a peremptory norm of general international law ( jus cogens) 10 from which no derogation is permitted, either by consent or by treaty. 11 To curtail the freedom of states to use force to settle international disputes, the use of force 7  in international law is prohibited by the UN Charter, according to which, in order to maintain international peace and security, all members of the United Nations are to settle their international disputes peacefully.
Even though the use of force is widely prohibited under international law, it may be justified under some exceptional conditions. Article 51 of the UN Charter states: 12 Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations until the Security Council has taken measures necessary to maintain international peace and security. Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security.
Article 51 is clear enough to allow a state's exercise of the right of self-defence only if it has been the victim of an 'armed attack'. As the International Court of Justice (ICJ) pointed out in the Oil Platforms case, the defending state 'must … show that its actions were necessary and proportional to the armed attack made on it and that the platforms were a legitimate military target open to attack in the exercise of self-defence'. 13 Some scholars and states have argued that self-defence is also the case when an 'imminent threat' of attack clearly exists. 14 Self-defence in that sense would be 'anticipatory: 15 the use of armed force by a state to repel an attacker before an actual armed attack has taken place, before the army of the enemy has crossed its border, and before the bombs of the enemy fall upon its territory. 16 Although Article 2(4) of the UN Charter restricts the use of armed force to the resolution of international disputes, it nevertheless recognises in Article 51 the 'inherent right' of states to act in self-defence. Proponents of anticipatory self-defence argue that it is consistent with Article 51 of the UN Charter if the evidence of a threat is compelling and the necessity to act is overwhelming, particularly in combat with armed non-state actors. 17 This particularly would be the case for the Turkish military intervention against Islamic State in Syria, 18 which is discussed in Section 4. 19 While this article is concerned with Turkey's recourse to armed force under international law, it is also worth noting that the Turkish Constitution (1982) establishes legal authority for the Turkish government to make the ad bellum decisions in conformity with the applicable rules of international law. The reference to jus ad bellum in the Turkish Constitution is in the first paragraph of Article 92, which states: 20 The power to authorise the declaration of a state of war in cases deemed legitimate by international law and except where required by international treaties to which Turkey is a party … to send the Turkish Armed Forces to foreign countries and to allow foreign armed forces to be stationed in Turkey, is vested in the Grand National Assembly of Turkey.
Although the phrase 'cases deemed legitimate by international law' refers to international law as the primary legal authority that legitimises the deployment of armed forces, attention should be paid to the phrase 'except where  18 For a discussion see George Brandis, 'The Right of Self-Defence Against Imminent Armed Attack in International Law', EJIL: Talk!, 25 May 2017, https://www.ejiltalk.org/the-right-of-selfdefence-against-imminent-armed-attack-in-international-law. 19 Turkey has recognised a right of self-defence against imminent attacks; see Letter dated 24 July 2015 from the Chargé d'Affaires a.i. of the Permanent Mission of Turkey to the United Nations addressed to the President of the Security Council (24 July 2015), UN Doc S/2015/563 (Letter, 24 July 2015). 20 Constitution of the Republic of Turkey, 7 November 1982, art 92, https://www.constituteproject.org/constitution/Turkey_2011?lang=en. It is worth being explicit in stating that the term 'legitimate' in Article 92 is intended to order all governmental bodies to respect the obligations arising out of international law in times of war and crisis. The accompanying commentary of Aslan Gündüz indicates that Article 92 limits the power of the legislature to declare war in the situation in which it is legitimate under international law to do so. The term 'international law' as used in Article 92 comprises both conventional and customary international law and restricts the power of the legislature to declare war in the situation in which it is legitimate under international law to do so. In other words, the Constitution orders the authorities, wherever required, to comply with international law. On this understanding, the phrase 'cases deemed legitimate by international law' obviously refers to the 'legal' conditions under which the use of force is permitted by conventional and customary international law: Aslan Gündüz, 'Eroding Concept of National Sovereignty: The Turkish Example ' (1991)  required by international treaties to which Turkey is a party'. The Grand National Assembly of Turkey, according to this provision, authorises the declaration of a state of war where required by international treaties to which Turkey is a party and, therefore, can send the Turkish Armed Forces to foreign territories. There can be no doubt that the phrase 'except where required by international treaties to which Turkey is a party' is an exception to the Grand National Assembly's authority and, thus, the right to use armed force would be implemented under the international treaties that regulate resorting to the use of armed force in foreign territories. The primary instances that might fall into this category are self-defence under Article 51 of the UN Charter and sending the Turkish Armed Forces to foreign territories under Article 5 of the North Atlantic Treaty Organisation (NATO), 21 to which Turkey is a party. In the same vein, Turkey has resorted to the use of armed force in foreign territories based on bilateral security or defence treaties. Therefore, it would be logical to explore how Turkey has applied this in practice. For the sake of clarity, the most convenient way to explore Turkey's practice relating to the use of force in line with bilateral treaties is to examine its justifications for using force on Syrian territory.

Use of force asserted in line with bilateral security or defence treaties
Overshadowing the earlier discussion, the Turkish authorities' interpretation and implementation of the phrase 'cases deemed legitimate by international law' contained in Article 92 of the Turkish Constitution is based on their own assessment of the situation. This is the case particularly where the Grand National Assembly of Turkey approved military operations in northern Syria as being subject to the bilateral security or defence treaties to which Turkey is a party. 22 In this section I will address Turkey's treaty-based justifications for military intervention in Syria, which focus directly on the source and validity of the territorial state's consent to intervention and the use of armed force on its territory. In particular, I consider the justifications for military intervention and 3.1. Background leading up to the bilateral security or defence treaties Turkey shares its longest common border (911 km) with Syria in the southeastern part of the country. The border between Turkey and Syria historically has been contentious and a major cause of tension between the two countries. Major reasons for the triggered tensions between the two countries over the years have been the joining of the Turkish and Arabic-speaking province of Hatay with Turkey in 1939; Turkey's damming of the Euphrates river as part of the Southeast Anatolia Project (SAP), which has long been criticised for its negative effects on the natural environment, cultural heritage, and the local population since the 1970s; 23 and the Syrian government's support of the PKK and its founding member and leader, Abdullah Öcalan, in 1980. However, following the Adana Security Agreement 24which was signed by Turkey and Syria on 20 October 1998, and which obliged the Syrian government to expel Abdullah Öcalan -Turkey and Syria have turned over a new leaf in their relations. To eliminate the existing tensions between the two countries and to stabilise the border region, the agreement entailed the following commitments: 25 As of now, Öcalan was not in Syria and he definitely will not be allowed to enter Syria; the PKK elements abroad will not be permitted to enter Syria; as of now, the PKK camps are not operational and definitely will not be allowed to become active; and many PKK members have been arrested and have been taken to court.
In 1998, when the Turkish and Syrian authorities discussed defusing the border tensions between the two countries, they made a joint commitment to cooperate in combating terrorism. According to the Adana Security Agreement and based on the principle of reciprocity, Syria would not permit any activity that emanates from its territory aimed at jeopardising the security and 23 Presumably, the SAP has been utilised to fight the PKK through using the Euphrates and Tigris as a bargaining chip to force both Syria and Iraq to cut their support to the PKK; see Arda Bilgen, , 20 October 1998, https://www.mfa.gov.tr/_p_statement-made-by-ismail-cem_-foreign-minister_-on-the-special-security-meeting-held-between-turkey-and-syria_br_ october-20_-1998_br__unofficial-translation___p_.en.mfa. stability of Turkey. It would not allow the supply of weapons, logistical material, financial support, or propaganda activities of the PKK on its territory. Syria recognised that the PKK was a terrorist organisation and had, alongside other terrorist organisations, prohibited all activities of the PKK and its affiliated organisations on its territory. It would not allow the PKK to establish camps and other facilities for training and shelter or commercial activities on its territory. Nor would it allow the PKK to use its territory for transit to third countries; it would take all necessary measures to prevent the leader of the PKK terrorist organisation from entering Syrian territory and would instruct its authorities at border points to that effect. 26 Turkey and Syria agreed upon a counter-terrorism strategy through which both sides needed to combat terrorism. Given that the main objective of the Adana Security Agreement is to fight the PKK and its extensions into Syrian territory, both sides agreed to establish certain mechanisms for the effective and transparent implementation of the measures mentioned above. 27 However, the Syrian government has not been effective enough in applying the major terms and conditions of the agreement. It was the main reason that led Turkey and Syria to reopen discussions over the Adana Security Agreement and sign a revised document, the Joint Cooperation Agreement, 28 in 2010. This agreement also includes security cooperation activities within the borders of both parties. In this context, each party will take effective security measures against terrorist acts, terrorist organisations and members of terrorist organisations on its territory. 29 3.2. Bilateral security or defence treaties as primary justifications for use of force against armed non-state actors Far more recently, following the tensions that arose in northern Syria in 2015, Vladimir Putin, President of the Russian Federation, stated that the Adana Security Agreement dealt with the fight against terrorism and it was the base that closed many issues in terms of ensuring Turkey's security on its southern borders. Mevlut Çavuşoğlu, Turkey's Minister of Foreign Affairs, interpreted this statement as the green light to move its military forces into Syria, mentioning that 'we think he [Putin]  Besides, the Adana agreement signed on 20 October 1998 by the Republic of Turkey and the Syrian Arab Republic constitutes a contractual basis for my country to fight all kinds of terrorism emanating from Syrian territory in its hideouts and in an effective and timely manner.
However, Article 4 of the Joint Cooperation Agreement is insufficient to justify the use of force on Syrian territory without the consent of the Syrian government. In other words, it does not authorise military intervention. More precisely, the amended document provides only for 'joint cooperation' in the fight against terrorist organisations, including the PKK and its extensions, as well as any other violent groups active in Syria and Turkey. 32 Both the Adana Security Agreement and the Joint Cooperation Agreement provide that the contracting parties will never allow any terrorist or other violent group to use their territory to violate their national security and stability. Importantly, the Joint Cooperation Agreement stipulates that both Syria and Turkey have committed to pursue all terrorist groups in perpetuity, and to take all necessary joint measures to a certain degree by identifying their resources and locations. 33 In essence, it appears that consent to take all necessary measures 34 to pursue terrorist groups is an independent justification for Turkey to use force against armed non-state actors, but nothing in the Adana Security Agreement or the Joint Cooperation Agreement addresses military intervention, territorial secession, a safety zone, unilateral invasion or occupation of the other contracting party's territory.
Moreover, even under the given bilateral treaties, Turkey's reliance on the use of force against an imminent threat of the PKK, the Kurdish People's Protection Units (Yekîneyên Parastina Gel (YPG)), and the Democratic Union Party intention of the contracting parties at the time of the negotiations. It is possible, therefore, to argue that although there are prominent examples of treaties that have authorised external military interventions accompanied by ad hoc consent at the time, 35 bilateral defence treaty regulations might potentially be abused by powerful states. 36 What is clear in the Syrian case is that the agreements in question do not authorise the use of force on the territory of the contracting parties; it appears clear, therefore, that intervention in Syria is an approach that stems from the broad interpretation of the key provisions of the Adana Security Agreement and the Joint Cooperation Agreement, according to which the contracting parties have agreed to take 'necessary measures' for certain purposes. 37 As a matter of principle, only the clear consent of a state to a particular act of armed force, if freely and properly given, can legitimise military intervention which otherwise would have been unlawful. 38 Returning to treaty-based military interventions, state practice indicates that although the use of force based on a bilateral treaty is one of the primary circumstances that may justify forcible military intervention, 39 the intervening state is prohibited from taking any action in violation of the traditional rules of international law that have been developed for the purpose of territorial protection. 40 In other words, even consent does not preclude the wrongfulness of any act of a state which is not in conformity with an obligation arising under a jus cogens norm of general international law. Although jus cogens norms do not render bilateral defence treaties invalid, the prospective unauthorised military intervention that falls far short of the principal objective of the relevant bilateral defence treaty is a decisive issue that would bring the international responsibility of the intervening state for violating the applicable rules of international law. Ultimately, any broad interpretation of bilateral treaties would invite stronger states to intervene by treaty in the affairs of the relatively weaker states, a practice that would be incompatible with the principles of the UN Charter. 41 Having said this, the violations committed by Kurdish fighters in the parts of northern Syria recaptured from Islamic State have been the motivating factor to open the door for Turkey to bring forward the bilateral counterterrorism agreements. As Amnesty International reported in February 2015, the YPG began to demolish villages and displace villagers after taking control of Rojava, the de facto autonomous region in northern Syria, which had been under the control of Islamic State. Most residents affected by the YPG's unlawful practices were Arabs and Turkmen; however, in some cases (including in the mixed town of Suluk), Kurdish residents were also barred by Kurdish fighters from returning to their homes. 42 As reported by the Syrian Network for Human Rights, the Syrian Democratic Forces (SDF), led by the YPG, carried out arbitrary arrests and enforced disappearances in areas under their control, targeting political activists and media journalists who opposed their policies, as well as carrying out arrests with the aim of forced conscription. They also detained civilians, including women and children. As a means of advancing in northern Syria, Kurdish fighters detained at least 2,705 individuals, largely of Arab and Turkmen descent, at the Turkish-Syrian border between March 2011 and March 2019. They justified their actions as being for the civilians' own protection. 43 In such circumstances it appears that the terrorist threat embodied by the Kurdish military advance in northern Syria compelled Turkey to resort to armed force, including cross-border operations, to suppress the threat, particularly in the aftermath of the activities of Islamic State. In the broad meaning of the phrase, one might argue that the fight against Islamic State's terrorist actions seems less of a rational explanation for Turkey's presence in northern Syria. What is clear, however, is that the measures Turkey has taken on Syrian territory stem from the Turkish authorities' interpretation of necessary 41 Reisman (n 35) 153. 42 Amnesty International, 'Syria: US Ally's Razing of Villages Amounts to War Crimes', 13 October 2015, https://www.amnesty.org/en/latest/news/2015/10/syria-us-allys-razing-of-villages-amountsto-war-crimes; Amnesty International, 'We Had Nowhere Else to Go: Forced Displacement and Demolitions in Northern Syria', 12 October 2015, https://www.amnestyusa.org/reports/we-hadnowhere-else-to-go-forced-displacement-and-demolitions-in-northern-syria. 43  counter-terrorism measures as a framework to safeguard their national security and stability. In practice, it seems that Syria's failure to comply with its obligation to counter terrorism has allowed Turkey to use force against the precedent groups in northern Syria as a last resort, by adopting a broad reading of the Adana Security Agreement and the Joint Cooperation Agreement based on its own assessment of the situation. Again, however, it does not justify military intervention on Syrian territory.
It is remarkable that the primary objective of the Adana Security Agreement was to repel the advances of terrorist groups in the Turkish-Syrian border area. Given that Turkish troops had already repelled the YPG and Islamic State fighters by seizing control of between 30 and 35 km of Syrian territory, their continued presence in Syria as an extension of treatybased intervention was contrary to the primary objective and purpose of both the Adana Security Agreement and the Joint Cooperation Agreement. Both agreements provide that Turkey has the right to repel terrorists and other violent groups from its borders so that they can no longer threaten its national security. Removing Kurdish fighters from approximately 35 km of Syrian territory adjacent to its borders indicates that Turkey has gone far beyond the purpose of the Adana Security Agreement, which in its Annex 4 44 confines any conceivable necessary Turkish security measures to an area 5 km deep into Syrian territory, stating 'the Syrian side understands that its failure to take the necessary measures and security duties, stated in this agreement, gives Turkey the right to take all necessary security measures within 5 km deep into Syrian territory'. 45 Ultimately, Turkey's continued military presence on Syrian territory would be considered a violation of Syria's territorial integrity as it has ignored the legitimacy and authority of the Syrian government on its territory. As a result, the latest action elicited a harsh response from the Syrian government, which sent identical letters dated 31 October 2019 to both the UN Secretary-General and the President of the UN Security Council. The letters stated that Turkey had occupied several Syrian villages, and its military aggression against the Syrian people continued unabated. 46 Having reaffirmed its territory's inviolability, sovereignty and integrity, Syria reiterated the government's unwavering determination to continue the war against terrorism and to liberate any territory, particularly in northern Syria, that terrorist groups continue to control.
One reasonable reading of this statement is that there is a lack of consensus and disagreement over the legal obligations of both contracting parties under the Adana Security Agreement, seemingly on the basis that they reject each other's approach to the issue of combating terrorism under the Adana 44 See Claus Kreß, 'A Collective Failure to Prevent Turkey's Operation "Peace Spring" and NATO's Silence on International Law', EJIL: Talk! 14 October 2019, https://www.ejiltalk.org/a-collective-failure-to-prevent-turkeys-operation-peace-spring-and-natos-silence-on-international-law. 45 Adana Security Agreement (n 24); see also Kreß (n 44). 46  Security Agreement, which has obviously led to inconsistent outcomes. A normative argument in such a situation is that any necessary actions taken by Turkey or Syria concerning any threats to the peace or acts of aggression would therefore meet the ad bellum criteria under the UN Charter. This simply stems from Article 103 of the UN Charter, which provides that in the event of a conflict between the obligations of the member states under the Charter and their obligations under any other international agreement, their obligations under the Charter shall prevail.
What is much clearer, however, is that neither the Adana Security Agreement nor the Joint Cooperation Agreement allows unilateral measures against armed non-state actors in the territory of the contracting party. Again, it is important to note that the treaties signed in both 1998 and 2011 require a 'joint operation' in the event of a terrorist attack by any violent armed group. This is perhaps why Turkey has then relied on the use of force in self-defence asserted in line with the unwilling or unable theory, alleging that Syria has not fulfilled the conditions of the Adana Security Agreement or the Joint Cooperation Agreement; thus the state of necessity has emerged, and Turkey has the right to take necessary measures unilaterally.

Use of force in self-defence asserted in line with the unwilling or unable theory
Terrorism is a phenomenon that has often brought to the fore the Turkish government's concerns about its national security. Tensions between Turkey and armed non-state actors based in neighbouring countries raise the question of whether Turkey has acted in compliance with the ad bellum criteria by using force against such actors in foreign territories. In its statements Turkey has relied on the unwillingness and inability of the Syrian government to prevent the threats of non-state actors emanating from its territory as a justification for military intervention in northern Syria. This part, initially, seeks to highlight whether the jus ad bellum paradigm supports the legality and effectiveness of the Turkish military intervention on Syrian territory based on the unwillingness or inability of the Syrian government. It then aims to answer whether and to what extent Turkey's justifications are sufficient to meet criteria for aggression or armed attack which can give Turkey a valid ad bellum basis to use force against imminent attacks.

Measures were taken because of the unwillingness or inability of the territorial state
Turkey's geopolitical position has made it more vulnerable to the significant cross-border security threats emanating from terrorist groups and other violent armed non-state actors, including the PKK, the YPG, the PYD, the Kurdish Democratic Confederalist political party established in 2003, and Islamic State based in Syria. Following the emergence of Islamic State, Turkey has suffered terrorist attacks not only in the Turkish-Syrian border area but also in its major cities, including Ankara and Istanbul. The targets of these attacks have almost always been the Turkish state, civilians or demonstrations. 47 Islamic State has repeatedly targeted Turkey. Regardless of the justifications for military intervention in foreign territories, the anxiety of the Turkish authorities regarding Islamic State's frequent attacks is understandable, and it is a vital issue for Turkey. As a result, although Turkey was part of the US-led coalition fighting Islamic State, the Turkish Armed Forces also unilaterally intervened in Syria to attempt to halt Islamic State's terrorist operations.
On 24 July 2015, Turkey sent the following letter to the President of the UN Security Council justifying its use of armed force against Islamic State in Syria: 48 With the emergence of Daesh, the threats from Syria gained new dimensions. Syria has become a haven for Daesh. This area is used by Daesh for training, planning, financing, and carrying out attacks across borders. Also, Security Council Resolutions 2170 (2014) and 2178 (2014) have underscored the threat posed by Islamic State and the resolve of the international community to combat Daesh. The terrorist attack that took the lives of 32 Turkish citizens in Suruç on 20 July 2015 reaffirms that Turkey is under a clear and imminent threat of continuing attack from Daesh. Most recently, on 23 July 2015, Daesh attacked the border military post in Elbeyli and killed a Turkish soldier. It is apparent that the regime in Syria is neither capable of nor willing to prevent these threats emanating from its territory, which clearly imperil the security of Turkey and the safety of its nationals. Individual and collective selfdefence is our inherent right under international law, as reflected in Article 51 of the UN Charter. On this basis, Turkey has initiated necessary and proportionate military actions against Daesh in Syria, including in coordination with individual members of the Global Coalition, to counter the terrorist threat and to safeguard its territory and citizens.
In its most basic form, the unwilling or unable theory is a situation in which 49 47 The conflict in Syria and the threat of Islamic State emanating from northern Syria have also been the major causes of displacement of the highest number of people and refugees in the world today. According to the UN Refugee Agency, in September 2018, 5.64 million Syrian refugees were registered in neighbouring countries. Turkey currently hosts over 3.6 million registered Syrian refugees: UN High Commissioner for Refugees, 'Refugees and Asylum Seekers in Turkey: 2019 Planning Summary', 15 February 2019, http://reporting.unhcr.org/sites/default/files/pdfsummaries/GA2019-Turkey-eng.pdf. 48 Letter, 24 July 2015 (n 19). 49  a state (the 'victim state') suffers an armed attack from [an armed nonstate actor] operating outside its territory and concludes that it is necessary to use force in self-defence to respond to the continuing threat that the group poses … If the territorial state is willing and able, the victim state may not use force in the territorial state, and the territorial state is expected to take the appropriate steps against the [armed non-state actor]. If the territorial state is unwilling or unable to take those steps, however, it is lawful for the victim state to use that level of force that is necessary (and proportional) to suppress the threat that the [armed non-state actor] poses.
As mentioned earlier, Turkey's initial attacks against Islamic State in January 2014 and its joining of the US-led coalition's armed operations under the unwilling or unable theory intensified the terrorist operations against them. This is a framework for extraterritorial self-defence, according to which using extraterritorial force against armed non-state actors depends on a determination that the relevant armed non-state actors perpetrated an armed attack against the victim state or other states, and the host state is unwilling or unable to suppress the threat. 50 Having relied on this theory, Turkey intervened in northern Syria to use force against Islamic State in self-defence against the threats it posed, as the Syrian regime was unwilling or unable to halt the threats emanating from its territory. 51 As their primary justification, the Turkish authorities declared that Syria, as the host state, had been unwilling or unable to control its territories, which were under the effective control of Islamic State and were being used as a base for its terrorist operations. 52 Therefore, to protect itself against these threats, the Turkish Armed Forces entered Syria to use force in self-defence. 53 Turkey deployed its armed forces on Syrian territory with neither the consent of the Syrian government nor UN Security Council authorisation. 54 Attribution and the Non-State Actor: Birth of the "Unable or Unwilling" Standard?' (2015) 75 Heidelberg Journal of International Law 460, 460-61. 50 Dinstein (n 4) 195, 201-03; see also Deeks (n 49). 51 Letter, 24 July 2015 (n 19). 52 It is important to note that Islamic State's armed attacks are not attributed to Syria as the Syrian government did not have 'effective control' over Islamic State and it has never authorised the group's activities. Therefore, the territorial state (Syria) cannot be held responsible for any violation of international law and for the threats and terrorist acts of armed non-state actors who are . 54 Turkey took a similar position by using force against the terrorists operating in northern Iraq between 1993 and 2003. Although Iraq protested against what it considered to be an act of aggression, it did not deny the right of Turkey to take coercive measures to the extent that this was strictly necessary to neutralise terrorist groups. While the legality of the Turkish intervention Nevertheless, the use of force based on the Syrian government's supposed unwillingness or inability to act for itself is still unacceptable. As Gill and Tibori-Szabó have argued, although the lack of feasible alternatives to selfdefence in the form of law enforcement or cooperation with the territorial state may stem from the refusal of the territorial state to exercise its obligation to halt the threat of armed non-state actors, this refusal does not itself give rise to the right of self-defence. 55 The targeted state may take action in selfdefence only if it is clear that the territorial state will not do so, and there are no other feasible alternatives to thwart the attack. However, no self-reliant unwilling or unable test replaces or supplants the principle of necessity, which remains the bedrock requirement for the exercise of self-defence. 56 While the necessity for self-defence arises from the combination of an ongoing or impending armed attack and the lack of feasible alternatives, no necessity for self-defence will arise if the territorial state undertakes effective measures to neutralise the threat of an armed attack by an armed non-state actor present on its territory. 57 Viewed from this perspective, it would be unfair to say that Syria has not fought against Islamic State or other terrorist groups within its territory, given that it has launched many operations against Islamic State and other terrorist targets, both unilaterally and with the comprehensive support of its regional and strategic allies, including Iran and Russia.
No one is allowed to use armed force against any armed group in foreign territories without the host state's consent or UN Security Council authorisation. While the inability or unwillingness of a state to prevent armed non-state actors from operating on its territory may be a factor in assessing the need to act in self-defence, 58 the issue remains controversial and continues to give rise to disagreements. It is worth noting here that the ICJ left this issue open in its decision in the Case concerning Armed Activities on the Territory of the Congo, when it stated that 'the Court has no need to respond to the contentions of the Parties as to whether and under what conditions contemporary international law provides for a right of self-defence against large-scale attacks by irregular forces'. 59 In effect, however, the Court has adopted a restrictive approach, 60 as it was apparently unwilling to accept that the operation of a terrorist group from the territory of the target state per se justifies the use of force in that state by a state allegedly acting in self-defence. 61 Therefore, the legality of the Turkish military presence on Syrian territory and its approach to extraterritorial self-defence against armed non-state actors based in foreign territories is contentious simply because it has moved beyond the contemporary ad bellum considerations. This is particularly obvious in the context of the indiscriminate attacks by the Turkish-backed armed forces of the Free Syrian Army and of Turkey's reliance on self-defence against attacks stemming from particular parts of Syrian territory invaded by the Turkish Armed Forces. In such a situation, self-defence would remain unlawful if the defence is invoked by the defending state against non-state actor attacks stemming from the territory occupied by the defending state. 62 Again, while some states (such as Turkey, Iran, the US and the UK) have explicitly or implicitly accepted the principle that defensive measures are permissible when the neighbouring state is unwilling or unable to prevent crossborder attacks, 63 using force in foreign territories based on the unwilling or unable theory remains highly controversial. This is especially so because it has not gained recognition as a customary rule, nor can it be found under conventional international law or acknowledged in the reasoning of an international tribunal. This is particularly evident from the Nicaragua case in which the ICJ explained that it has not recognised the legal effectiveness of the unwilling or unable theory and pointed out that 'there is no rule in customary international law permitting another state to exercise the right of selfdefence based on its own assessment of the situation'. 64 Moreover, an armed attack is still the primary requirement for using force in self-defence, and military actions in self-defence against the armed nonstate actors who have perpetrated an attack should be compatible with the general rules of jus ad bellum, including necessity and proportionality. 65 As a matter of jus ad bellum, however, the question remains whether armed nonstate actors can be the authors behind an armed attack. Again, what is rather clear is the fact that the unwilling or unable theory is viewed as a consideration that has no basis in customary international law. 66 By conferring on every state the power unilaterally to implement its conception of the necessities of the war against terror, the unwilling-and-unable theory bypasses, if not simply ignores, this core provision, along with the entire collective security system established by the UN Charter. This is precisely why most states have not accepted the unwilling or unable theory, 67 either in the Syrian case or more generally. 68 An important issue to be highlighted further in this section is that the Security Council is the only UN body with authorisation to determine the existence of any threat to the victim state, and it should make an appropriate decision regarding the unwillingness or inability of the territorial state. While the Security Council should determine the existence of any threat to the victim state and make an appropriate decision regarding the issue, the unwillingness or inability of the Syrian government to suppress the threat posed by Islamic State has been a controversial matter for the international community.
That being said, Turkey initially resorted to military action against Islamic State in Syria based on the unwilling or unable theory. In the letter to the UN Security Council, Turkey referred to its inherent right of individual and collective self-defence against Islamic State because of the Syrian government's unwillingness or inability to combat Islamic State, which posed a serious threat to Turkey's national security. In the meantime, however, it appears clear that Turkey also used the Islamic State crisis as an opportunity to expand its operations against the PKK, YPG and PYD. The idea is that the victories of these groups in the region could enhance the consolidation of Kurdish territories, which may lead to the creation of a Kurdish corridor. 69 This is probably the most likely hypothesis as to why Turkey intervened in Syria: it is concerned that the area of northern Syria controlled by the YPG could become part of a potential Kurdish state. However, as Detter has argued, this can be categorised as a 'patronising intervention' 70 by Turkey, which refers to the actions in northern Syria as being legitimised for its national security reasons; this, at least in international law, confers no legitimising effect at all, although in political terms it may explain the reason for its actions. 71 The axis around which the Turkish military intervention in Syria revolves, therefore, is directly related to terrorist attacks as Turkey's major domestic issue. The growing threat of Islamic State and Kurdish militants provided a foundation for Turkey to use force against both. Following the outbreak of the non-international armed conflict in Syria in 2011, Turkey endeavoured to protect its border from illegal migration. However, the invasion and occupation of northern Syria by Islamic State and YPG militants compelled Turkey to take drastic measures to protect its border with Syria. As a security-enhancing measure, Turkey started to construct a wall along its entire 911 km-long south-eastern border with Syria, and it deployed its troops and equipment near the border in 2015. Nonetheless, this strategy did not completely succeed as a protective measure. The Istanbul nightclub massacre 72 and the Russian ambassador's assassination in Ankara 73 were the most notable pieces of evidence that indicated that the wall policy and the deployment of Turkish troops to the border per se were unable to protect Turkey from the cross-border threats of Islamic State and the YPG. With all that said, these threats led the Turkish military to cross the border to counteract the threats posed by Islamic State and to prevent the PKK and its extensions in Syria from establishing themselves west of the Euphrates along Turkey's borders. 74 In the wake of the above-mentioned threats, Turkey launched three major operations in northern Syria. The Turkish Armed Forces carried out the first operation, Operation Euphrates Shield, in August 2016 in the triangle between Azaz, Jarablus and al-Bab in northern Syria. 75 The second operation, Operation Olive Branch, was launched in January 2018 in northern Syria to protect Turkey's national security. 76 The final operation, Operation Peace Spring, began on 9 October 2019 to eradicate the terrorist threat. 77 The Turkish authorities justified the operations as self-defence measures against Islamic State 78 and Kurdish militants in Syria, which would ensure Turkey's continued existence and security by clearing terrorists from the region. 79 Turkey stated, in its last statement, that the ultimate goal of Operation Peace Spring, 80 [which was launched] in the face of multi-dimensional terror threats emanating from Syria against our national security, [was] to ensure the security of our borders, to neutralise terrorists in the region and to save the Syrian people from the oppression of terrorists … The operation [was] being conducted on the basis of international law, in accordance with our right of self-defence under Article 51 of the UN Charter and resolutions of the UN Security Council on the fight against terrorism.
This last point inevitably raises the key conceptual question of whether aggression or an armed attack was valid as the main ad bellum criterion which allowed Turkey to use force in self-defence. [t]he PKK/PYD/YPG units close to Turkish borders in the north-east of Syria, continue to be a source of direct and imminent threat as they opened harassment fire on Turkish border posts, by also using snipers and advanced weaponry such as anti-tank guided missiles.
Having released this statement, the justifications propounded by Turkey bring to the forefront two crucial arguments regarding the legality of the intervention in northern Syria. Initially, it appears that the most controversial point in the letter is the imminent threat allegation, which scholars have often criticised as an invalid argument for justifying the use of force in self-defence. This issue has also divided the UN member states. They have disagreed explicitly over whether they have the right to use anticipatory armed force to defend themselves against imminent threats and whether they have the right to use it preventively to defend themselves against latent or nonimminent threats. 86 Note in this respect that Turkey has expressed its concerns, stating that the weapons given by the United States to the YPG to use against Islamic State in Syria are ultimately finding their way to the PKK for use against Turkey 87 as a result of the weakness of Islamic State on Syrian territory. 88 However, even if the authorising factors, as explained above, robustly justify Turkey's position in using force against the YPG and Islamic State as a last resort, its military response needs to have complied with the basic requirements of self-defence, including necessity and proportionality, contained in Article 51 of the UN Charter as the core principles of international law regulating jus ad bellum. As the ICJ reaffirmed in the Nicaragua case, the lawfulness of any response to an attack depends on the observance of these criteria. 89 Otherwise, any form of military operation in self-defence that does not respect the conditions mentioned might be regarded as a violation of Article 2(4) of the UN Charter.
By itself, an imminent attack is one where the attacker has committed a particularly aggressive course of action from which they will not desist unless there was some kind of intervention in the causal chain, such as using force in self-defence. 90 Relatedly, the UN Secretary-General's report based on the High-Level Panel Report on Threats, Challenges and Change offers an objective and verifiable basis for the use of force against imminent threats when it states: 91 The language of [Article 51] is restrictive … However, a threatened state, according to long established international law, can take military action as long as the threatened attack is imminent, no other means would deflect it and the action is proportionate.
However, this is still disputed and the paramount challenge is in determining the type of threat that might be considered equivalent to an imminent attack 92 and the criteria that would indicate imminence. 93 There remains a contradiction in that the wording of Article 51 is ambiguous, and it is not clear to what extent it prohibits the adverse effects of the use of force in self-defence against armed non-state actors based on another state's territory. Article 51 remains open to reckless and broad interpretations, which might be inconsistent with the primary objective of the right of self-defence: to ensure the national security, territorial integrity, and political independence of states against aggression and armed attacks. However, it seems unlikely that a broad interpretation of Article 51 can change the ordinary meaning of using armed force in self-defence. As Haque has rightly pointed out, the vast majority of parties to the UN Charter have said and done nothing to establish an agreement with broad interpretations of Article 51 to justify the use of armed force against the threat of armed non-state actors in foreign territories. 94 This, in itself, reduces the weight of subsequent state practice as a supplementary means of interpreting the UN Charter. 95 To be precise, the use of force is legally permitted only where there is aggression or an armed attack against the state. This ad bellum criterion needs to be met as the central pillar of the right to self-defence under the UN Charter. According to the ICJ, only the gravest forms of the use of force constitute an armed attack, which can trigger self-defence. 96 That is to say, less grave forms of an armed force or a series of low-scale attacks will not be answerable by lawful self-defence even though they violate Article 2(4) of the UN Charter. 97 A complex problem, however, remains because of the lack of a clear definition of 'armed attack'. The issue should rather be whether the attack produced serious consequences, epitomised by territorial intrusions, human casualties, or considerable destruction of property. 98 This is, perhaps, a threshold for using force in self-defence, according to which Turkey has relied on the ad bellum criteria by explicitly considering certain actions as armed attacks. This is, of course, an elementary point but is not a matter that is of central importance in this case. Turkey already believes that an armed attack has occurred or is imminent. More importantly, the armed force may be used in self-defence only when it is necessary to end or avert an armed attack. On this basis, Turkey might rely on the right to use armed force on Syrian territory if the attacks have crossed the threshold and all peaceful means of ending or averting the attack have been exhausted. 99 After all, Turkey justified its military operations on Syrian territory in its letter to the UN Security Council by referring to Resolutions 2170 (2014) and 2178 (2014) in which it characterised Islamic State's cross-border terrorist actions as armed attacks, reaffirming that Turkey is under a clear and imminent threat of continued attack. 100 This, however, is a matter of controversy simply because Islamic State's sporadic attacks have not substantiated Turkey's claim of an armed attack, and it is difficult to equate them to the gravest uses of force. 101 What is true is that Turkey has directly operated and justified its military operations under the ad bellum rules because of the lack of certain definitions of 'armed attack' under jus ad bellum. This has allowed Turkey to benefit from the ambiguity of the definition of both an armed attack and an imminent attack as it relates to anticipatory self-defence in response to an imminent armed attack as a controversial form of pre-attack self-defence. 102 While, on the one hand, the ICJ has left open the issue of the lawfulness of self-defence against the threat of an imminent armed attack, 103 it has been the consistent position of the UN General Assembly and some member states that imminent threats are fully covered by Article 51 of the UN Charter, which safeguards the inherent right of sovereign states to defend themselves against the attack. 104 Lawyers have long recognised that this covers imminent attacks and attacks that have already happened. 105 This has been the position of the United Kingdom, the United States and many other states over the years that selfdefence against an imminent armed attack is lawful under international law, 106 which is also supported by some scholars. 107 Note, however, that existing international law does not offer strong guidelines on the use of force in anticipatory self-defence against the threat of an imminent armed attack by armed non-state actors, as discussed earlier. It certainly cannot be disputed that the current manifestations of the use of force by armed non-state actors, with or without the support of a state, call for a liberal construction of the requirement of armed attack in Article 51 of the UN Charter, where states facing existential threats from such elements are to be in a position to defend themselves. It is impossible, however, to turn a blind eye to the dangers of allowing a state, from its unilateral assessment and determination of 'imminent threat', to breach the territorial integrity of another state in the guise of acting in self-defence. This is the position of the ICJ in the DRC v Uganda case, under which the Court rejected an expansion of the right of self-defence to include a right to anticipatory self-defence against the threat of an armed attack by armed non-state actors. 108 In that sense, the best one can say is that Turkey has moved beyond the purpose of Article 51 of the UN Charter simply because nothing in Article 51 addresses military intervention, a unilateral invasion, or occupation of a foreign territory as measures against threats of armed non-state actors.

Conclusion
To address the Turkish practice in the implementation of jus ad bellum, this article explored the current debates on jus ad bellum to bring clarity to the law-fulness of Turkey's extraterritorial self-defence operations. First, it was discussed that the Turkish military intervention in Syria is an extension of its constitutional approach to jus ad bellum, where it has relied on both selfdefence and treaty-based justifications for military intervention in foreign territories. In other words, we have seen that Turkey has relied on two basic scenarios in justifying the use of armed force on Syrian territory. The first was the use of force in line with bilateral security or defence treaties signed between Turkey and Syria. It was explained, however, that none of the given treaties allows unilateral measures against armed non-state actors on the territory of the contracting party. The key point in that regard was that both the Adana Security Agreement and the Joint Cooperation Agreement require only a 'joint operation' in the case of a terrorist attack by any violent armed group. This was precisely the reason why Turkey then relied on the use of force in self-defence under Article 51 of the UN Charter and asserted in line with the unwilling or unable theory as the second scenario, alleging that Syria has not fulfilled the conditions of the given treaties.
However, as explained earlier, Turkey has moved beyond the purpose of both the bilateral treaties and Article 51 of the UN Charter simply because nothing in either the given treaties or Article 51 addresses military intervention, a unilateral invasion, or occupation of Syrian territory as measures against imminent threats of armed non-state actors. Concerning Article 92 of the Turkish Constitution, which considers the use of force in cases deemed legitimate by international law, Turkey must exercise that right responsibly in compliance with its general obligations under conventional and customary international law. Recall that in this context states may use armed force only in response to an armed attack and that there is no evidence in existing international law to support anticipatory self-defence against the threat of an imminent armed attack by armed non-state actors on foreign territory.
Finally, this article has sought to show that Turkey's military presence in northern Syria, based on the Adana Security Agreement, has lost legitimacy as the primary objective of the agreement is to repel the advances of terrorist groups across Turkish borders. As Turkey has already repelled Kurdish and Islamic State fighters by seizing control of a buffer zone some 30 to 35 km deep, the continued presence on Syrian territory is contrary to the main purposes of the Adana Security Agreement and the Joint Cooperation Agreement. A continued military presence on Syrian territory would, therefore, be considered a violation of Syria's territorial integrity, usurping the authority of the Syrian government within its borders. In other words, military presence on a territory not fully sanctioned by valid agreement is a criterion that can help to identify military occupation. 109 In sum, given that the exclusive purpose of self-defence actions is to halt and repel an armed attack, seizing control of part of Syria and the consequent displacement of local civilians may be an example of excessive force 110 used by Turkey.