Empires rise and fall, sometimes very fast, like the Mongol Empire and the Soviet Union, both of which lasted a hundred years or less. Hitler’s empire, if we can call the Third Reich an empire, lasted twelve years.
The Roman Empire ruled the world for about a thousand years. It died from within, when further expansion was so expensive, that it could no longer be sustained. As the bureaucracy and complexity grew, so did the gulf between rich and poor. As the surrounding countries got invaded, annexed or conquered and turned into provinces, and as taxes were collected, every denarius of taxes was spent on administration and the military to keep the provinces. The empire became more and more complex and expensive to run, and the military budget was out of control.
The inequality of wealth became obscene. In the first century, all the land from today’s Germany to today’s Iraq (Euphrates) was owned by just 2,000 grotesquely rich people. Tax evasion and corruption was rampant. The rich found loopholes and the poor had to prop up the military and administration. Nero was living in splendor. The middle class, once the backbone of the empire, declined because it was squeezed by the aristocracy and by the endless needs of the ever-growing hordes of the poor. The empire was eating itself from within.
The rest, as we say, is history.