Sir John Glubb was a British author and lecturer, who served 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, 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.
Glubb's summary at the end of the essay:
(a) We do not learn from history because our studies are brief and prejudiced.
(b) In a surprising manner, 250 years emerges as the average length of national greatness.
(c) This average has not varied for 3,000 years. Does it represent ten generations?
(d) The stages of the rise and fall of great nations seem to be:
The Age of Pioneers (outburst)
The Age of Conquests
The Age of Commerce
The Age of Afﬂuence
The Age of Intellect
The Age of Decadence.
(e) Decadence is marked by:
An inﬂux of foreigners
The Welfare State
A weakening of religion.
(f) Decadence is due to:
Too long a period of wealth and power
Love of money
The loss of a sense of duty.
(g) The life histories of great states are amazingly similar, and are due to internal factors.
(h) Their falls are diverse, because they are largely the result of external causes.
(i) History should be taught as the history of the human race, though of course with emphasis on the history of the student’s own country. _PDF Download of Sir John Glubb's Essay on Fate of Empires
Useful supplementary reading:
Decline of the West by Oswald Spengler
The Evolution of Civilisations by Carroll Quigley
History of the Decline and Fall of the Roman Empire Vol. 1 by Edward Gibbon
Historians often disagree over details -- both large and small. That leaves it up to each of us to learn what we can, and to make up our own minds as to the lessons that we can apply from history to more modern times.