Larger and larger political states are required to support the creation of larger and more advanced war machines. When spears ruled the battlefield, a relatively small infrastructure was needed to mass produce the weapons of war. More advanced weapons such as crossbows, chariots, catapults, and armor require a more advanced infrastructure to develop and mass produce.

Borders of nations expand until there is no more room for expansion either due to a natural boundary or another nation's boundary. If the other nation is drastically weaker as well as filled with valuable resources, it will be consumed by its neighbor. When nations achieve an extremely large size, it is not worth trying to conquer one another. In the animal world, intimidating threats serve as a deterrent for combat because even if you are clearly bigger and stronger the little guy still has sharp teeth and claws and might get lucky enough to severely wound you. With substantially sized and well resourced nations, it is to the point where even if you conquer them, you will be severely hurt by it and so it is not worth it.