“The decline of Rome was the natural and inevitable effect of immoderate greatness.”Edward Gibbon · The History of the Decline and Fall of the Roman Empire · 1776
Gibbon analyzed hundreds of years of Roman history and showed the decline came gradually, not suddenly. The army grew expensive to maintain, leaders changed too often, and the borders became harder to defend. Outside attacks were, in fact, the final step of a much older internal weakness.
Why it matters Modern historians often compare today's great powers to Rome, using this same pattern of slow decline.