New research suggests that willingness to fight and die for a group is motivated by ‘identity fusion’ - a visceral sense of ‘oneness’ with the group. Identity fusion has been documented among revolutionary insurgents, football hooligans, ritual participants, and soldiers. Individuals fused to a group are willing to take great risks to defend it, even to lay down their lives if necessary. This research provides important insights into the within-group causes of intergroup violence and suggests also some novel approaches to the prevention and suppression of warfare.