"The decline of Rome was the natural and inevitable effect of immoderate greatness. Prosperity ripened the principle of decay; the causes of destruction multiplied with the extent of conquest; and as soon as time or accident had removed the artificial supports, the stupendous fabric yielded to the pressure of its own weight."
Edward Gibbon --The Decline and Fall of the Roman Empire
After ruling much of the known world for centuries, Rome fell due to a number of factors that, historians believe, would not have been fatal in isolation, but that proved terminal in combination. Military overspending and overreach, an untenable economic system, and currency debasement all played a role. As has been well documented, the Roman emperors attempted to distract the populace from the increasingly dire reality of their situation by providing bread and circuses. But entertainments could not stop the nation-state from yielding to the pressure of its own weight.
Conquest & Overreach
"The decay of trade and industry was not a cause of Rome's fall. There was a decline in agriculture and land was withdrawn from cultivation, in some cases on a very large scale, sometimes as a direct result of barbarian invasions. However, the chief cause of the agricultural decline was high taxation on the marginal land, driving it out of cultivation. Taxation was spurred by the huge military budget and was thus "indirectly' the result of the barbarian invasion."
Arthur Ferrill -- The Fall of the Roman Empire: The Military Explanation
The Roman Empire's economy was based on the plunder of conquered territories. As the empire expanded, it installed remote military garrisons to maintain control and increasingly relied on Germanic mercenaries to man those garrisons.
Ultimately, as its territorial expansion waned and began to contract, less and less booty became available to support the empire's widespread ambitions and domestic economy. The outsourcing of the military and the cultural dilution from the bloated empire led to lethargy, complacency, and decadence amongst the formerly self-reliant and hard-working Roman citizenry.
In the modern context, as the only major power whose productive capacity was not destroyed during World War II, the American Empire emerged from the ashes of that conflict.
The parallels with Rome do not repeat, but they do rhyme.
Rather than plunder, the U.S. used its unique status to dictate terms that made the U.S. dollar the world's de facto reserve currency and positioned its robust new manufacturing sector to supply the world with the cars, machinery, appliances, and electronics it so desperately needed. The U.S. trade surplus with the nations of the world led to escalating U.S. wealth and prosperity.
Simply, in the 20th century, the U.S. became an empire in all but name.
Bread and Circuses