To answer the question, we first must define war. According to the Merriam-Webster dictionary, war is the "state of usually open and declared armed hostile conflict between states or nations." That definition helps us narrow down when humans invented war. If we're talking about states or nations, we must focus on early civilizations. Before civilization, all humans were tribal and at least somewhat nomadic. It was only after we developed agriculture and settled down that we could build the resources needed for war.