Five experiments investigated the war contagion phenomenon in the context of international relations, hypothesizing that reminders of past inter- (but not intra-) state war will increase support for future, unrelated interstate violence. After being reminded of the Korean War as an interstate rather than intrastate conflict, South Koreans showed stronger support for violent responses to new, unrelated interstate tensions (Study 1). Replicating this war contagion effect among Americans, we demonstrated that it was mediated by heightened perceived threat from, and negative images of, a fictitious country unrelated to the past war (Study 2), and moderated by national glorification (Study 3). Study 4, using another international conflict in the U.S. history, provided further conceptual replication. Finally, Study 5 included a baseline in addition to the inter- versus intrastate manipulation, yielding further support for the generalized effect of past interstate war reminders on preferences for aggressive approaches to new interstate tensions.