The scientific study of war has largely ignored necessary conditions for war onset. Conflict scholars have previously identified alliances as a mechanism that brings about the initial expansion of war but have not examined whether it is a prerequisite for large wars. We argue that wars diffuse into multiparty wars only in the presence of alliances. In other words, in the absence of any alliance ties, war would not include more than two parties. We put forth a theoretical rationale for this relationship and conduct a series of tests on both dyadic and multiparty wars between 1816 and 2007. These tests provide support for our hypothesis, suggesting that alliances are a virtual necessary condition for multiparty wars: the larger the war, the more likely alliances are a necessary condition.