At the height of their power they had vast riches and powerful armies dwarfed any other power in the world. People of all races and nations admired and wanted to be apart of those empires. And those that did not, were crushed.
History has shown over and over again that all empires of this unrivaled power are never ever conquered, rather they suffer from the rot within.
Both the Roman and the American Empires collapsed internally as they descended into decadence, debt and death. Most historians typically cite the collapse of empires as a result of a constant state of foreign wars, growing bureaucracy of the state, and an increasingly ignorant masses deluded by bread and circuses. The combination of expenses and depreciating productivity that leads to the debasement of the currency and ultimately the end of the empire. While scholars would argue that story, we feel that there is a far larger issue that humanity is afraid to come to terms with.