To examine whether support for force in war is influenced by the proportionality of combatants' force and subjects' group membership with combatants, 215 American college students were exposed to a news transcript that factorially varied an Invading Country (U.S., England, U.S.S.R.), Invader's Tactics (moderate violence, high violence) and Defender's Tactics (nonviolence, moderate violence). Results supported a proportionality hypothesis whereby invader's force was deemed inappropriate and deserving of a prison term to the extent that invader's force was disproportionately violent to defender's force. Also, group membership moderated this effect such that force by the Soviet Union was rated as more inappropriate than identical force by the U.S. or England. Discussion addresses the role of procedural rules of conflict in support for intergroup conflict, and evidence in the current study that the moderating effect of group membership was more consistent with Sherif & Sherif's (1969) Realistic Conflict Theory than Tajfel & Turner's (1979) Social Identity Theory.