From AlFin 2100:
Sir John Glubb was a British author and lecturer, who was decorated for his service in the Royal Engineers in WWI, and was commander of the Jordan Arab Legion from 1939 to 1956. His famous and succinct essay, The Fate of Empires and Search for Survival (PDF), looks at the lifespan of empires from their origins to their eventual decline.
Glubb estimates that most empires do not last longer than roughly 250 years, with many of them lasting much shorter periods of time. He describes many of the stages of empire, and many of the reasons why they break down and eventually disappear.
As seen in Glubb's image above, most of the world's great empires lasted no longer than 250 years. Glubb looks at the Roman Republic and the Roman Empire as two different empires, based upon their distinct forms of government.
One of the reasons for decline of empire described by Glubb is the influx of masses of people from outside cultures, religions, and ethnic groups, who are different from the core populations making up the founders and conquering peoples who brought about the original empire....MORE
