Gentili’s conceptualization of war as a conflict between states attempted to limit the legitimacy of war to external wars only, thus precluding the legitimacy of civil wars. It reflected both the emergence of sovereign states and the vision of international law as a law among polities rather than individuals. The conceptualization of war as a dispute settlement mechanism among polities rather than a punishment for breach of the law of nations and the idea of the bilateral justice of war humanized the conduct of warfare and the content of peace treaties. The idea of perfect war excluded brigandage, piracy, and civil wars from its purview. Some scholars have suggested that perfect war had a dark side, legitimizing imperial expansion. Others have cautioned that Gentili explicitly opposed imperial expansion rather adopting anti-imperialist stances. This article suggests that these ambivalent readings of the Gentilian oeuvre reflect the ambivalence of the early modern law of nations. Under the early modern law of nations, aggression for the sake of empire was clearly unjust; nonetheless, imperial expansion took place. Whereas ‘a law which many transgress[ed] [wa]s nonetheless a law’, there was a wide divide between theory and practice.