First, this chapter provides a general framework of international law at the time of the League of Nations, taking into account the practice of international law; second, a sample of the international legal literature of the period concerning a number of key and recurrent topics is offered; finally, a few historical treatments of international law that were recounted at the time are briefly summarised to show that certain developments in practice and in theory had repercussions also in the historical conceptualisations of international law.

The treatment of the law of war and neutrality during the interwar era was dominated by two trends. Scepticism was voiced at the relevance and utility of these fields in light of poor compliance during the First World War and the emergence of jus contra bellum. This did not prevent others from engaging with the most pressing issues, including the challenges posed by technological developments (submarines, asphyxiating gases and military aviation); the protection of civilian populations, especially against the risks of aerial bombardment; and the need to expand the application of the law of war in cases of insurrection and civil war. Later events show that the international community did not address these challenges satisfactorily. Initiatives failed due to ‘technological determinism’ and concern for national interest. However, several developments clearly foreshadowed the transition from a law of coexistence to one of co-operation that would come to fruition with the Geneva Conventions of 1949. Indeed, many of the fundamental tenets of modern international humanitarian law emerged, as the law of war began to afford more importance to the rights of the individual.

In this chapter, the authors frame the interwar period as instrumental for the institutionalisation of international dispute settlement, with respect to both the establishment of institutions and the development of new applicable law. The chapter focuses on the institutions, but equally emphasises the foundational principles which govern the field, with the principle of consent at the forefront; with all their characteristic features and limitations, such principles are conspicuous and remain valid today. The chapter gives context to the creation of the Permanent Court of Arbitration (PCA) and the subsequent establishment and the main features of the Permanent Court; last but not least, it pays homage to the mixed arbitral tribunals, with their impressive machinery and cases decided. The interwar period was undoubtedly a time of experimentation, but it would be naïve to believe that it has come to an end: experiments remain ongoing.

This chapter offers a concise analysis of the legal framework on the protection of foreign investment at the time of the League of Nations, in many respects a rather turbulent period. Such legal framework was essentially composed of a rudimentary network of very heterogeneous bilateral treaties, some basic customary rules, and some broad general principles largely based on the notions of justice and equity. Many fundamental questions remain fiercely disputed, and insurmountable divergences between states to a large extent frustrated the attempts to codify the international rules on the protection of foreigners and their property. Yet the legacy of this period should not be underestimated. From a substantive perspective, state practice already demonstrated a certain convergence on some basic rules on the treatment of foreign investors, while some legal claims put forward by some states – including the so-called Hull formula concerning compensation for expropriation – failed to muster the general acceptance necessary to become legally binding rules. Regarding the settlement of investment-related disputes, an embryonic role for investors started to surface in this period.

Both the theoretical and practical roots of contemporary preventive diplomacy, peacekeeping and peacemaking trace back to the League of Nations. Starting with the Vilna dispute in 1920, the League organised collective action on several occasions, achieving varying degrees of success in different parts of the world. Although the League was ultimately unsuccessful in its overarching objective of preventing another global conflict, its results in preventing local conflicts from erupting or escalating have been unduly neglected. Furthermore, although a theoretical framework for preventive diplomacy, peacekeeping and peacemaking would not be developed until many decades after the League had dissolved, its basic contours may be found already in the work and reflections of the organisation’s bodies and functionaries. This chapter analyses the League’s pioneering work in an attempt to present the organisation’s own contribution and better understand these phenomena in light of their early iterations.

Colonies and mandates, along with protectorates, belong to the wider group of ‘dependent’ territories. Colonies were under the total control of a foreign power which decided all aspects of the administrative, executive and legislative organisation. Public international law was mainly relevant for slavery, forced labour and ‘open-door’ policies. The mandates system was certainly inspired by colonialism, especially in the eyes of contemporaries, for whom colonialism was the ‘white man’s burden’ for the benefit of ‘uncivilised peoples’. However, it also had fundamental structural differences: their purpose – the ‘civilising mission’ – and the triangular relationship (League of Nations, territory, mandate), stand in sharp contrast to the colonial institution. In addition, the mandatory power was not the holder of sovereignty over the mandated territory. The triangular relationship refers particularly to the control that is supposed to embody it. The control exercised by the League marks a notable difference from the colonial system, establishing for the first time in the history of international relations a sophisticated form of indirect international administration of territories.

This chapter deals with questions of sovereignty, territory and jurisdiction during the League of Nations era. It discusses how the concept of sovereignty developed until the League era and how it was understood then. Questions of territory and jurisdiction are closely linked with sovereignty, but, given the immense scope of this topic, it will only be considered as far as it affects the central substance of the chapter. This general exposition of the concept of sovereignty will be followed by an explication how the interwar period saw the emergence of, first, its general principle of horizontal protection of the territorial and jurisdictional aspect of states by international law; second, structured exceptions to this principle qua its vertical limitation of sovereignty through the League system; and third, curious cases where the territorial and jurisdictional powers of states had to be reconciled with other innovative legal principles such as human rights and self-determination. These explications will be illustrated and substantiated by a selection of the relevant cases decided by the Permanent Court of International Justice and other judicial bodies.

The League of Nations was the first permanent international organisation with a general mandate. Its establishment is widely regarded as having had a significant, if elusive, impact upon international law, which became centred on international institutions. These three aspects of the League – its permanence, the generality of its mandate, and the ’institutional turn’ it brought to international law – lie at the heart of the assumed significance of the League for contemporary international lawyers. They are regarded as the League’s principal innovations and central components of its legacy, often without much interrogation and rarely subject to sustained analysis. This chapter offers analysis and interrogation to nuance claims about the League’s innovations. It presents the League as an institution whose grand designs often failed, but which innovated quietly and gradually. Above all, it shifts the focus away from the perceived ’breakthrough’ of 1919, and highlights the evolutionary nature of the League, which adapted throughout its life.

The distinction between international and domestic law plays an essential function in the establishment of international law as an autonomous order. During the lifespan of the League, this distinction was contested by scholars and judges in increasingly sophisticated ways. This process culminated in the debate between monists and dualists. However, the formal conceptual foundations of this debate meant that it failed to take account of the way that bureaucrats, officials and experts at the League adopted equally sophisticated normative strategies as part of the ‘experiment of international administration’. Such strategies, which lay at the heart of attempts to promote the ‘well-being and development’ of peoples subject to mandatory rule and foster co-operation across social and economic fields, creatively transformed conventional understandings of the relationship between international law and domestic law. This chapter juxtaposes these simultaneous, countervailing trends of formalisation and deformalisation in international law and administration to offer fresh insight into the crucial formative period in the history of the distinction between international law and municipal law.

The interwar years are the shortest period covered in these volumes, and perhaps the most intense and eventful one. So many things happened in the short course of this quarter of a century that it would deserve an entire Cambridge History series of its own. Unlike other volumes, whose temporal boundaries are harder to establish, this one does not pose a problem. It is firmly locked between the iron parentheses of two devastating catastrophes, if such a pleonasm may be allowed.

The Old Regime period in which war proved the norm and peace the exception witnessed the development of the modern law of nations. Questions of international law assumed a new urgency as did the status of diplomatic agents. By this time the existence of permanent embassies could still be deplored but no longer questioned, and diplomatic immunity could not be disputed, reinforced as it was by a body of precedent and tradition. This period witnessed first the expansion and later the contraction of diplomatic privilege. European aristocratic society reinforced diplomatic privilege, for the status of the ambassador was inextricably intertwined with that of the ruler. The explosive expansion of diplomats and their staff led many theorists, such as Grotius and Vattel, to analyse the evolving conventions, such as the importance of the civil immunity of the ambassador and the liability of the embassy staff. Practice tended to reinforce privileges identified as personal, that is, attached to the ambassador himself. Of these the exemption from criminal liability was perhaps the most important. Among territorially defined privileges, the right of asylum and the notorious right of quarter were first expanded and later either limited or eliminated.