Paul Krugman has written two columns in the last few days comparing the Pax Romana and the Pax Americana, and finding that both were sustained by soft power. He’s right; empires are initially won by force, but they can only endure if the conquered parties accept that the conquering party is a superior civilization in some way. Once that disappears, decline is inevitable.
The two were also different, however. The Roman Empire was created for the direct purpose of exploiting the native populations, and was maintained by respect for a common culture. The Pax Americana, on the other hand, did not involve much self-interested military conquest, and was sustained more by respect for America’s political institutions and sense of restraint than by any love for our culture, although that does exist in some ways that have little to do with our government.
The Roman Empire largely fell because it had to stop growing; as a result, there was no more land and booty to be distributed to the population. It appears that the Pax Americana will die because our present government simply no longer accepts its burdens, and would much rather do business with ambitious strongmen than with our erstwhile allies.