Friendship is a concept often evoked on the international stage and which has a long history in international thought. Despite falling to the sidelines for some time, it is experiencing a resurgence of interest. This paper sets out the position of friendship in the international thought of Hugo Grotius. I show that Grotius develops a law of friendship (amicitiae lex) which he incorporates into his just war theory. I argue that he makes a conceptual distinction between allies (socii) and friends (amici). The distinction concerns the underlying legal bases upon which they are founded. I therefore argue that we can map this distinction onto another of Grotius’s distinctions, that between perfect and imperfect rights. This is an important point because an infringement of a perfect right can result in war, but an infringement of an imperfect right cannot. Additionally, I highlight the supererogatory character to which Grotius accords friendship. I conclude by arguing that friendship is an important concept in Grotius’s international thought. I also suggest that Grotius is an instructive figure not only for those engaged in friendship’s revival but also for those concerned with the identity and relations of states.
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