Gabriel Kolko sums it up

The dilemma the U.S. has had for a half-century is that the priorities it must impose on its budget and its plans have never guided its actual behavior and action. It has always believed, as well it should, that Europe and its control would determine the future of world power. But it has fought in Korea, Vietnam, and now Iraq – the so-called "Third World" in general – where the stakes of power were much smaller. Its priorities were specific, focused on specific nations, but they also set the United States the task of guiding or controlling the entire world – which is a very big place and has proven time and again to be far beyond American resources and power. In most of those places in the Third World where it massively employed its power directly, it has lost, and its military might has been ineffective. Local proxies have been corrupt and venal in most nations where the U.S. has relied upon them. The cost, both in financial terms and the eventual alienation of the American public, has been monumental.