The purpose of this chapter is to clarify some of the disagreements that have arisen regarding Gentili’s ideas by closely examining Gentili’s use of sources in De iure belli (DIB). More specifically, through this focus on sources, the chapter seeks to resituate Gentili’s DIB beyond the immediate legal debates that have conventionally framed its analysis, and to place the text within the broader political thought of the period. Gentili’s text, the chapter claims, is particularly noteworthy in its attempt to straddle the legal and political debates of its epoch, looking beyond legal sources and turning to various contemporary political writers in what was quite a remarkable move at the time.

Trying to find intellectual space between theology and raison d’état, the Protestant Hugo Grotius aimed to derive an autonomous sphere of legality from the natural inclination of humans to follow rules even when that might appear against their immediate self-interest. The rules he wrote into De iure belli ac pacis (On the Rights of War and Peace) combined a contractual justification of absolutism with a system of individual rights that he believed underwrote the spectacularly successful commercial practices of his native Holland. This rule-system would accommodate the actual practices of European warfare and government while also appealing to the better natures of European rulers. Unlike Gentili, Grotius viewed law as a moral science that engaged both the external behaviour as well as the conscience of those who govern.

The Eastern Roman Empire represents a prototype of a Christian state both in terms of its political philosophy of Church–state relations and in terms of a political philosophy regarding legal recognition of other states – both Christian and non-Christian. The process through which Byzantium recognized emerging states in Europe and beyond became an important element of international legal order as this dominant Christian empire addressed the most critical question of political legitimacy: how can a political order established by Christ recognize other political communities that challenge its territory? As a political community established by Christ and driven by the rule of law, the Empire navigated its international relations following a very specific theological program that reflected its attitude toward its neighbors. This program held that any political project should be an imitation of the Byzantine Christ-given political community and through such process it would come closer in affinity to the Empire and become integrated into its Christian commonwealth.

What to focus on in an intellectual history of ius gentium et naturae for a volume on the relations between international law and Christianities? For centuries, (international) law and Christian theology maintained intensive and complex relations, which it is impossible to do justice to within the scope of this chapter. With the more recent “turn to history” in international legal scholarship, discussions of the relationship between ius gentium et naturae and Christianity generally center on secularization and/or empire. For obvious reasons both sets of histories deal with early modernity – the time that the so-called Respublica Christiana or Holy Roman Empire was profoundly affected by Reformations, gradually fragmented, and religious and theological fights were part of the politics of the newly emerging European nation-states.