When do states ally against a common threat despite ideological differences? This question presents a puzzle for two prominent theories of international relations. Realists expect that ideology animosity is no barrier to alliance formation, while ideological arguments expect that it is an insuperable obstacle. Mark L. Haas’s answer to this important and relevant puzzle provides new insights for scholars and policymakers to consider.

First, Haas argues that external threat is necessary but insufficient for ideological opponents to ally (20). In addition to an external threat pushing together two countries that ideological disagreements would otherwise pull apart, leaders must perceive low domestic political risks and a favorable ideological configuration in the international system (50). If leaders expect that security cooperation will embolden domestic revolutionaries and endanger their hold on power, they will not risk a “frenemy” alliance (24). Leaders must also believe that one country is their greatest ideological and material threat or that they can wedge apart states with similar ideologies as an “ideological outsider” (39).

Haas then uses three cases to examine his argument, providing new insight on a failed alliance between France, the UK, and USSR before World War II, the timing of US-China cooperation in the 19