Intergroup conflict is a persistent companion of the human existence. Why do individuals engage in intergroup conflict as often as they do? We propose that groups’ tendencies to present intergroup conflict as the default option and individuals’ tendencies to disproportionately choose default options fuel individual participation in intergroup conflict. Three experiments (total N = 893) that used incentivized economic games found support for this hypothesis. Designating intergroup conflict as the default option significantly increased individual participation in conflict relative to a no-default condition and to designating other behavioral options as defaults. The effects of defaults on intergroup conflict generalized across different social identities and levels of group identification. Our findings explain the stickiness of conflict and identify choice architecture as a potential solution: changing existing defaults can redirect intergroup behavior. We discuss promising directions for future research on the psychological mechanisms underlying these effects.