3.1. Background leading up to the bilateral security or defence treaties

Turkey shares its longest common border (911 km) with Syria in the south-eastern 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;Footnote 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 AgreementFootnote ²⁴ – which 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:Footnote ²⁵

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 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.Footnote 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.Footnote 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,Footnote 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.Footnote 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] referred to this [Adana Security Agreement] implying that Turkey can intervene in [Syria]’.Footnote ³⁰ Ultimately, Turkey extended the scope of both the original Adana Security Agreement and the Joint Cooperation Agreement to the terrorist actions of Islamic State. Again, in its letter dated 9 October 2019 to the UN Security Council, Turkey notified the Council:Footnote ³¹

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.Footnote 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.Footnote ³³ In essence, it appears that consent to take all necessary measuresFootnote 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 (Partiya Yekîtiya Demokrat (PYD)) units close to Turkish borders in the north-east of Syria lies far ahead of the purpose of Article 51 of the UN Charter. Put differently, the Turkish military intervention in northern Syria does not serve the purpose of either the Adana Security Agreement and the Joint Cooperation Agreement or Article 51 of the UN Charter. It is difficult, therefore, to rely on the Joint Cooperation Agreement or the Adana Security Agreement to grant permission for military intervention as a necessary measure in fighting against terrorist groups. It does not even seem to have been the 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,Footnote ³⁵ bilateral defence treaty regulations might potentially be abused by powerful states.Footnote 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.Footnote 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.Footnote 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,Footnote 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.Footnote 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.Footnote 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 counter-terrorism 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.Footnote 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.Footnote 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 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 treaty-based 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 4Footnote 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’.Footnote 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.Footnote 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 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.

4.1. 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.Footnote 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:Footnote ⁴⁸

In its most basic form, the unwilling or unable theory is a situation in whichFootnote ⁴⁹

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.Footnote 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.Footnote 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.Footnote ⁵² Therefore, to protect itself against these threats, the Turkish Armed Forces entered Syria to use force in self-defence.Footnote ⁵³ Turkey deployed its armed forces on Syrian territory with neither the consent of the Syrian government nor UN Security Council authorisation.Footnote ⁵⁴ 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 self-defence 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.Footnote ⁵⁵ The targeted state may take action in self-defence 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.Footnote 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.Footnote 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,Footnote 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’.Footnote ⁵⁹ In effect, however, the Court has adopted a restrictive approach,Footnote 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.Footnote 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.Footnote 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 cross-border attacks,Footnote 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 self-defence based on its own assessment of the situation’.Footnote 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 non-state actors who have perpetrated an attack should be compatible with the general rules of jus ad bellum, including necessity and proportionality.Footnote ⁶⁵ As a matter of jus ad bellum, however, the question remains whether armed non-state 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.Footnote 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,Footnote ⁶⁷ either in the Syrian case or more generally.Footnote 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.Footnote 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’Footnote 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.Footnote 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 massacreFootnote ⁷² and the Russian ambassador's assassination in AnkaraFootnote 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.Footnote 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.Footnote ⁷⁵ The second operation, Operation Olive Branch, was launched in January 2018 in northern Syria to protect Turkey's national security.Footnote ⁷⁶ The final operation, Operation Peace Spring, began on 9 October 2019 to eradicate the terrorist threat.Footnote ⁷⁷ The Turkish authorities justified the operations as self-defence measures against Islamic StateFootnote ⁷⁸ and Kurdish militants in Syria, which would ensure Turkey's continued existence and security by clearing terrorists from the region.Footnote ⁷⁹

Turkey stated, in its last statement, that the ultimate goal of Operation Peace Spring,Footnote ⁸⁰

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.

4.2. Self-defence against imminent armed attacks

Particularly distinct reasons have compelled Turkey to use force against Kurdish fighters in northern Syria, especially under Operation Peace Spring. According to Decision 2018/3775 E., 2018/5600 KFootnote ⁸¹ of the Sixteenth Criminal Division of the Court of Cassation (Supreme Court of Appeals of Turkey), Islamic State and the PKK/YPG/PYD have organised numerous terrorist attacks against Turkey.Footnote ⁸² The major examples are listed below:

1. (a) Terrorist attacks launched by Islamic State: Bomb attack in Sultanahmet (6 January 2015); bomb attack on the People's Democratic Party (Halkların Demokratik Partisi (HDP)) in Adana and Mersin (18 May 2015); the Suruç attack (20 July 2015); the Ankara explosion (10 October 2015); the Taksim attack (19 March 2016); the Gaziantep Şehitkâmil attack (1 May 2016); the Atatürk Airport attack (28 June 2016); the attack on a wedding in Gaziantep Şahinbey District (20 August 2016), and the Ortakoy Reina armed attack (1 January 2016).Footnote ⁸³

2. (b) Terrorist attacks launched by the PKK/YPG/PYD: the Sabiha Gökçen Airport attack (23 December 2015); the Ankara Military Service vehicle attack (18 February 2016); the Ankara Kizilay attack (13 March 2016); the Beşiktaş attack (10 December 2016), and the Kayseri attack (17 December 2016).Footnote ⁸⁴

More specifically, the Court of Cassation has considered Islamic State and Kurdish groups active in both Turkey and Syria to be equal in terms of their political ideologies and their threat to Turkey's national security and stability. Having compared the facts of the two sets of cases, the Court emphasised that the PKK, PYD, YPG and Kurdistan Freedom Hawks (or Teyrebazen Azadiya Kurdistan (TAK)) are the same for Turkey in that they pursue the common goal of creating a self-governing Kurdish state through organising terrorist attacks against Turkey in the north-west of the country. The same ideology regarding Islamic State and Al-Qaeda could be determined as being the same as that of the PKK, YPG, PYD and TAK. In that sense, the foregoing attacks are all claimed to be cumulative attacks against Turkey. In its letter of 9 October 2019 to the UN Security Council, Turkey notified the Council thatFootnote ⁸⁵

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 non-imminent threats.Footnote 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 TurkeyFootnote ⁸⁷ as a result of the weakness of Islamic State on Syrian territory.Footnote 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.Footnote 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.Footnote 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:Footnote ⁹¹

However, this is still disputed and the paramount challenge is in determining the type of threat that might be considered equivalent to an imminent attackFootnote ⁹² and the criteria that would indicate imminence.Footnote 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.Footnote ⁹⁴ This, in itself, reduces the weight of subsequent state practice as a supplementary means of interpreting the UN Charter.Footnote 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.Footnote 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.Footnote 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.Footnote 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.Footnote 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.Footnote 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.Footnote 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.Footnote 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,Footnote 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.Footnote ¹⁰⁴ Lawyers have long recognised that this covers imminent attacks and attacks that have already happened.Footnote ¹⁰⁵ This has been the position of the United Kingdom, the United States and many other states over the years that self-defence against an imminent armed attack is lawful under international law,Footnote ¹⁰⁶ which is also supported by some scholars.Footnote 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.Footnote 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.