The events of 9/11 set in motion a massive reordering of U.S. policy. We propose that the American public's response to this redirection in policy derives, in part, from ethnocentrism. Ethnocentrism—“prejudice, broadly conceived”—refers to the commonplace human tendency to partition the social world into virtuous ingroups and nefarious outgroups. Support for the war on terrorism, undertaken against a strange and shadowy enemy, should hold special appeal for Americans with an ethnocentric turn of mind. To see if this is so, we analyze the panel component of the 2000–2002 National Election Study. We find that ethnocentrism powerfully underwrites support for the war on terrorism, across a variety of tests and specifications, and the strength of the relationship between ethnocentrism and opinion is influenced in part by the extraordinary events of 9/11. Ethnocentrism is easily found among Americans, but its relevance and potency for politics depends, we suggest, upon circumstance.