The division between the Roman Republic and Roman Empire is purely arbitrary, as the position of Rome as a Mediterranean superpower and then hegemon over Western Europe and the Mediterranean was, despite numerous civil and foreign wars where Roman authority was temporarily lost over some areas, essentially uninterrupted from the end of the Second Punic War to the middle fourth century AD. Rome never met a setback she did not recover from, reimposing her preeminent position, stretching from the defeat of Hannibal outside Carthage at the end of the 3rd century BC, to the death of Julian in 363 AD. That's over 550 years, not 250.
Declaring that the United States was an empire from literally 1776 is pure idiocy. The United States was not a great power until ~1890, that to today is 130 years, not 250.
The Habsburgs were a great power from the 1400s and a superpower from the early 1500s up to World War I, again, significantly longer than 250 years.
The French were a European superpower and then a global empire from the end of the 1400s to 1940, despite numerous defeats they never lost their ability to drive European events until Hitler. Over 400 years.
More examples can be provided, of course. The original Assyrian empire. The New Kingdom of Egypt, almost 500 years. Multiple Chinese dynasties that were great empires for longer than 400 years.
Buddy I just keep embarrassing you and it's fun as hell, never stop.