This is the first book-length study of the role that friendship plays in diplomacy and international politics. Through an examination of a vast amount of sources ranging from diplomatic letters and bilateral treaties, to poems and philosophical treatises, it analyses how friendship has been talked about and practised in pre-modern political orders and modern systems of international relations. The study highlights how instrumental friendship was for describing and legitimising a range of political and legal engagements with foreign countries and nations. It emphasises contractual and political aspects in diplomatic friendship based on the idea of utility. It is these functions of the concept that help the world stick together when collective institutions are either embryonic or no more.