Toward the end of the Roman Empire, war was the only way Romans could keep up their level of consumption. It’s probably the same with most empires, particularly as they die. Easy access to cheap resources requires crushing anyone who might block that supply with needs of their own. It is part of the role of any Emperor or President of an empire to distract the people from what is really happening by casting our aggression into a defense of the homeland, and their resistance into a threat to our very existence.