Why is it that international ideological enemies—states governed by leaders engaged in deep disputes about preferred domestic institutions and values—are sometimes able to overcome their ideological differences and ally to counter shared threats, and sometimes they are not? Alliances among ideological enemies confronting a common foe are unlike coalitions among ideologically similar states facing comparable threats. Members of these alliances are perpetually torn by two sets of powerful contending forces. Shared material threats push these states together, while the effects of ideological differences pull them apart. To predict when ideological enemies are and are not likely to ally in the pursuit of common interests, it is necessary to know which of these contending forces is likely to dominate at a particular time. The values of two ideological variables beyond that of ideological enmity play the key role in determining outcomes: (1) states’ susceptibility to major domestic ideological changes and (2) the nature of the ideological differences among countries. Similar levels of ideological enmity and material threats will have vastly different effects on leaders’ alliance policies as the values of these additional ideological variables alter.