This paper proposes that war can be understood only by examining the human prohibition of violence. As prohibitions and allowances construct society, warfare is a major constructor. Men fight each other; we need therefore to examine this along the horizontal axis of lateral relations. The prohibition is traced back to the annihilation of the toddler's identity as “the baby” when a new sibling takes its place. The mother forbids the murderous wishes of the toddler. The result is an inner warring or clinical depression exemplified by Hamlet's iconic melancholia and—for boys—legitimate warfare. This is sharply divided from the murderous illegal violence from which it arose and has been split off and dissociated but must therefore psychically accompany it. The construction of two genders and of warfare's contribution to the creation of society are intrinsically related.