In its growth phase, it is easy for the empire to subjugate new territory. Resources extracted from conquered lands are used to fuel further expansion. The more resources the empire acquires, the easier it becomes to win battles against smaller adversaries. The army is the elite.
After a while the elite doesn’t need to fight anymore. Instead, mercenaries are recruited at the frontier of the empire. The proceeds of conquest are used to finance the elite’s lifestyle. Mercenaries are awarded citizenship for their service.
The elite becomes complacent and incompetent. Limits are reached and external growth stops. Resources now have to be extracted from the empire’s fixed territory. The subjugation process turns inwards. Up to a point taxes can feed the elite. But fewer and fewer soldiers are willing to fight for no gain. It becomes harder to defend the frontier.
Being a citizen of the empire loses its appeal. Ethnic lines of former conquered people re-emerge as their interests start to diverge from the interests of the elite. Slaves rebell. The empires descends into civil war and fractures.
Would a global empire escape this fate? Or would it crumble under its own corruption and incompetence?