It's definitely not obvious, given that many, many gregarious species may certainly have inter-group clashes and skirmishes at territory boundaries but no full-scale war. Animals in general avoid violence between conspecifics, for the obvious reason that it's rarely worth the risk of being hurt unless you're very sure you're going to win. Dying for your group is something you almost never see outside eusocial species. Never mind dying in your prime reproductive age!