"Empires do not last, and their ends are usually unpleasant. Americans like me, born before World War II, have personal knowledge--in some cases, personal experience--of the collapse of at least six empires: those of Nazi Germany, imperial Japan, Great Britain, France, the Netherlands, and the Soviet Union. If one includes all of the twentieth century, three major empires came tumbling down--the Chinese, Austro-Hungarian, and Ottoman. A combination of Imperial overstretch, rigid economic institutions, and an inability to reform weakened all these empires, leaving them fatally vulnerable in the face of disastrous wars, many of which the empires themselves invited. There is no reason to think that an American empire will not go the same way--and for the same reasons. If efforts at globalization delayed the beginnings of that collapse for a while, the shift to militarism and imperialism settles the issue."
Chalmers Johnson "The Sorrows of Empire"