One feature associated with democratic governance is frequent leadership turnover. While the ease of replacing leaders improves accountability, it may impede the ability of democracies to make credible long-term international commitments. Using newly collected data that identify cases in which leaders who derive their support from different domestic interests come to power, we evaluate the effects of changes in domestic political leadership on one important aspect of foreign policy—decisions to maintain military alliances. An analysis covering bilateral alliances between 1919 and 2001 reveals that changes in societal supporting coalitions in nondemocratic states are associated with decisions to abrogate alliances prior to their scheduled end dates, but changes in societal supporting coalitions in democracies have no effect on the probability of premature alliance termination. We conclude that international cooperation is sensitive to changes in core supporting coalitions, but that this effect is moderated by democratic political institutions.