A central concern for Law of Nations early modern scholars was the body of norms that regulated the alliances with friends and enemies. Regardless of whether whom we consider the father of international law to be, all the early ius gentium treaties devoted energy to this juridico-theological concern. And yet, by the time in which ‘modern international law’ emerged in the early twentieth century, questions of friendship and martial leagues are nowhere to be found in the legal lexicon. It seems, however, difficult to claim that alliances have disappeared from interpolity relations today. In this essay, I revisit the intellectual vocabulary used by Hugo Grotius in the De jure belli ac pacis (1625) on the law and practice of (un)equal alliances. For Grotius, as a firsthand witness of the emergence of a nominally post-imperial world of equal sovereigns, can teach us plenty about the legal consequences of friendship by treaty.
