When do leaders fear the domestic repercussions of revolutions abroad, and how does the prospect of such revolutionary waves affect international affairs? I argue that the fear of contagion is not primarily derived from the infecting agent - whether the revolutionary state serves as a model or acts as a platform. Instead, the fear of contagion is largely driven by the characteristics of the host, namely the presence of significant preexisting opposition groups in the host of the same character as the revolution. And I argue that when leaders fear revolution spreading, it will have a discernible effect on patterns of international cooperation and conflict: they will be hostile towards the revolutionary state and they will align with other states that face the same threat, sometimes in contrast to geopolitical pressures. I examine the reaction to the democratic revolutions under the ancien r�gime (the American Revolution and the Dutch Patriot Revolt), the wave of liberal revolutions in Europe in 1820-1, and the Iranian Islamist Revolution in the Middle East. The findings not only address the international effects of revolutions, but also the larger issues of when, why, and to what extent ideological differences between states affect international relations and how domestic instability interacts with international politics
