The American melting pot ideal works against the longevity of the American empire. From The Zman at theburningplatform.com:
During the Cold War, few thought of the Soviet Union as an empire. At least the claim was not part of propaganda campaign against them. The communists, of course, liked to hurl around the term “western imperialists”, but no one thought of the West or America as an empire. That was just part of the rhetoric used in the ideological war fought between the two sides in the third world. Empires fight to conquer land. East and West in the Cold War fought to win hearts and minds.
Looking back, it is easier to see that the Soviet Union was an empire, because it is so obvious that America is an empire. We are also seeing more parallels between the two sides as the American empire enters its end phase. Like the Soviets at the end, the American empire is run by a collection of ossified geezers, clinging to a past for no other reason than they have no future. What happened to the Soviet Union thirty years ago is now happening to the American empire.
An important similarity is both sides were cosmopolitan in their structure, organizing their people around a set of ideas. Traditional empires were organized around the conquering people, who imposed their will on those they conquered. Both the Russian and the American empires sought to assimilate the people brought into their orbit through ideology, rather than force. The captive people of both empires participated in the maintenance and defense of the empire, often as equals.