Because 'military history' is awful
The main problem with ‘military history’ is that the very term implies armies, and navies, and state organization. A better term might be ‘conflict history,’ which covers all forms of human conflict in whole, on a global scale. ‘Conflict history’ includes human conflict for all of time, at any scale. Cooperative aggression is a universal human experience, a social activity that every society shares to some degree.
Consider some of the post-apocalyptic stories in our current culture. What are the Walking Dead? They are the living characters in a Hobbesian world of war, not the zombies. In stories of a backwards future, the world often resembles Hobbes’s war of all against all, which he posits as the theoretical natural state from which the state emerged. Chiefdoms, kingdoms, city-nations, and nation-states evolved as systems to manage conflict. This is a testable hypothesis and we have plenty of evidence to consider.
But wait! In the human past, we succeeded through cooperation in non-violent endeavors at least 99 percent of the time. Are we not a social animal? Of course we are. This is how cities across the world started erecting defensive walls in the mid to late 4th Millennium BC. Causes and moral lessons are not the province of polemology. Humans will always fight about something, somewhere, because it is human of us to do so.