Group violence, despite much study, remains enigmatic. Its forms are numerous, its proximate causes myriad, and the interrelation of its forms and proximate causes poorly understood. We review its evolution, including preadaptations and selected propensities, and its putative environmental and psychological triggers. We then reconsider one of its forms, ethnoreligious violence, in light of recent discoveries in the behavioral and brain sciences. We find ethnoreligious violence to be characterized by identity fusion and by manipulation of religious traditions, symbols, and systems. We conclude by examining the confluence of causes and characteristics before and during Yugoslavia’s wars of disintegration.