In this paper, I investigate one of the most puzzling normative claims in international relations: ‘might makes right’. I first distinguish the version of this claim that interests me from alternatives with which it is often conflated, including a positive, causal one. I then explain why it is worth probing. Most significantly, ‘might makes right’ helps to elucidate criticism of social-fact-independent ‘ideal theory’ in political philosophy, including the global justice literature. Morally speaking, rights—or natural rights—are impervious to power. Yet Grotian scholars have often implied that, once instantiated in law, ‘might makes right’ might serve a valuable function given the fact of anarchy, which renders infeasible full compliance with natural morality through lack of enforcement. What argument, though, could be offered in its support? To address this question, I offer an interpretation of Grotius’s thought that draws on the theory of the second best. Grotius maintained that states have moral rights against aggression. Yet he also maintained that minimizing aggression where its potential cannot be wholly eliminated positively necessitates that aggressors be at least sometimes conceded legal rights.