Why American global hegemony?

Digby at Hullaballoo, inspired by discussion of Oliver Stone's Untold History series, asks when can we end the empire?

I've not yet seen any of Untold History, in large part because I have mixed feelings about Stone's relationship to history. He strikes me as a vivid example of a certain kind of incoherent leftist antinomianism which knows that something is rotten in Denmark but cannot offer a plausible theory about what — and doesn't really feel a need to. Stone will tell you that he's Just Asking Questions, and I think that's a valid project, but he also implicitly hints that he's offering answers when he isn't, which is at best sloppy and at worst outright irresponsible. Consider his JFK, which is a marvel of filmmaking ... but offered us a confused mashup of several different half-baked Kennedy assassination conspiracy theories. I have a sweet tooth for JFK assassination theories myself, but Stone's film isn't helping.

So I don't quite share Digby's enthusiasm for Stone, but I like the long post it drew from her, which hinges around this observation:

But even if it's absolutely true that the Soviet threat required two generation's worth of global military build up, it's also certainly true that one would have expected the period since 1989 to be one of withdrawal from empire. And that has not happened.

Why not?

Well that's the real question, isn't it? Why? The how and the who is just scenery for the public.
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Keeps 'em guessing like some kind of parlor game, prevents ’em from asking the most important question, why?
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Who benefited? Who has the power to cover it up? Who?

I'm just asking questions.