A basic truth about empires is that they have life spans just as people have life spans. They have a "breakout" or birth, a period of rapid growth and development (and heaven help anyone who is in the way!), eventually reaching a period of maturity with trade routes laid down, a single governing structure and administrative system, a single language and culture with sets of expectations for most of their citizens (or defining ethos), and a set of recognized defining achievements.
Then things begin to go off the rails little by little. They grow comfortable and complacent. Money and its trappings replace actual achievement and advancing the public good as the summum bonum. Both elites and masses get lazy and entitled (though not in the same ways, of course). They want the results of continued civilization but are losing sight of the work that went into building it.
They begin to lie to themselves about their mounting problems, which *may* be being caused by a system that requires growth whatever the cost. The lies grow legs. They start to dominate entire institutions, which grow further structures to protect the lies from any significant challenge. This includes intellectuals turning inwards to various academic-type micro-specializations. If they had significant founding principles, these are either perverted or forgotten.
Decadence ensues, which may include fomenting irrational wars outside their borders; it may include irrational monetary or other economic policies (such as undermining their own manufacturing base so that a tiny corporate elite can profit), or the embrace of other destructive tendencies (the list is potentially quite long).
Decadence always precedes either invasion from outside, if there is another expansionist power that perceives accelerating weakness and vulnerability, or just collapse from within as the society as a whole loses its collective will to live. Or both at once.
This is the final reality of empires. They always fall. Without exception. One of the lies we Anglo-Americans have been telling ourselves for quite some time now is that we are somehow the exception (Ha! We're the Exceptional Nation), that it is onward and upward forever ... to the stars! ... despite lying our way into wars of choice that are ultimately all to control the world's remaining oil supply, building an economic system designed to redistribute wealth upward based entirely on accumulated and rapidly growing debt, a political system the utter dysfunction of which grows more evident with each "election," and a culture now taken over by "wokesters" who want to censor / cancel whatever gives the slightest offense to their delicate, flowerlike senses of entitlement as they *worsen* everything they claim to oppose.