The present article seeks to draw attention to and explicate the concept of friendship in a discipline which has long ignored it: International Relations. It examines the ways in which major political thinkers and international treaties addressed the concept in the process of the emergence of the Westphalia state system. The article traces correlative changes between the shift from vertical to horizontal friendship and the emergence of internal and external princely sovereignty which signified the new era in international politics. It argues that the recognition of formally equal statuses of political friends prepared the grounds for the regime of external sovereignty. It also suggests that friendship was a key concept describing political order, included or not in the friend/enemy antithesis, in early Modernity. The subsequent ambiguity of the modern concept of friendship in international politics springs from its constant reinterpretation in the context of royalist and republican ideological polemics.