Three experiments investigated the phenomenon of war contagion in the context of international relations, hypothesizing that past inter- (but not intra-) state war will facilitate future, unrelated interstate war. Americans showed stronger support for violent responses to new, unrelated interstate tensions after being reminded of an historical war between the U.S. and another state, as compared to an historical domestic war within the U.S. (Study 1). This war contagion effect was mediated by heightened perceived threat from, and negative images of, a fictitious country unrelated to the past war, indicating a generalized effect of past interstate war on perceived threat/images from any foreign country. The war contagion effect was further moderated by national glorification (Study 2). Largely replicating these effects with an additional baseline condition, Study 3 yielded further support for the generalized effect of past interstate war on perceived threat and images, this time with a real third-party country.