The sixteenth and early seventeenth centuries marked a deep crisis of the international political and legal order of Europe, caused by the Reformation, the emergence of some strong composite monarchies and the discovery of the New World. The chapter maps how the law of nations began to emerge as a new paradigm for the governance of Europe under whose wings rulers, diplomats and scholars attempted to advance claims to an exclusive jurisdiction over international relations by sovereign princes and republics. As such, the ‘law of nations’ functioned as a lever, an argument for power in a period of great clashes between centralising governments, opposing confessions, and regional and local elites, rather than representing a reality. The ultimate success by governments in several important states at the end of the Renaissance was facilitated to a great extent by the patrimonial and transactional nature of the states that allowed to include old, autonomous powers in the machinery of state.

In the great treatises of Gentili and Grotius, the law regulating the ending of war and the restoration were discussed as the third, chronological and logical part of the laws of war. Their conceptualisation of peace was premised on their conceptualisation of war either as an instrument for the vindication of justice or as the settlement of disputes about right. The chapter explains how these theories, and in particular the latter conception, reflected the practices of peacemaking in peace treaties well, focusing on such major issues as the temporal and spatial dimensions of peace, territorial disputes and the place of communitarian interests in a logic that was geared towards the settlement and appeasement of bilateral disputes over dynastic rights.

This chapter explores the justification and legitimisation of war and ‘imperfect’ uses of force both in legal scholarship and diplomatic practice. The Grotian synthesis of ‘just’ and ‘formal’ war entered mainstream scholarship and reached its full explanatory force in the work of Vattel. The resilience of just war in the face of its impracticality among sovereign state in scholarship can, among other, be explained by the fact that the subtle interplay of two conceptions of law in fields of legal application - , gelled well with diplomatic practice. Whereas states applied the jus in bello and jus post bellum with regards to claims to the justice of the war, the just war doctrine remained a common discourse for the justification of resort to war and force and added an instrument to the toolbox of alliances diplomacy.

The Old Regime period saw the highpoint of the role of peace treaties in the political and legal ordering of Christian Europe. Whereas the peace instrument by and large continued to adhere to the legal logic of the settlement of disputes about right between pairs of belligerents, important peace treaties entered into the constitutional fabric of Europe through the workings of multilateral alliances and general peace conferences and through the networking of treaties in different fashions. The encompassing nature of warfare also led to a phase of the growth of the length and legal complexity of peace treaties, particularly during the seventeenth century. The eighteenth century in turn saw a trend towards the standardisation of peace treaty clauses, marking the emergence of an elaborate body of peacemaking lore, or even law.

The sixth volume in The Cambridge History of International Law covers the developments of inter- and transnational laws in early modern Europe (1492–1660). The preface explains how recent revisionism of traditional state- and Eurocentric views on the history of international law impacts the study of this subject and how this is reflected in the volume.

By the middle of the seventeenth century, a category of sovereign princes and polities had succeeded in monopolising jurisdiction over external relations and the internal machinery of government that allows to speak of sovereign state. The Old Regime saw the further emergence, in governmental and diplomatic practice as well as in learned writings of the paradigm of the law of nations as the preserve of sovereigns. As legal practice and literature, it also expanded in scope and mass to new regulatory fields such as the law of the sea, maritime warfare, neutrality or dispute settlement. The great treatises on the law of nations of the middle of the eighteenth century fleshed out the dualist system of law of nature and of nations that formed one of the intellectual backbones to Grotius’ work into an elaborate framework of the governance of international relations inside Christian Europe and for its imperial expansion outside.