Daniel Larison, in the course of arguing - rightly, I think - that Woodrow Wilson's foreign-policy legacy was far more disastrous than George W. Bush's will prove to be, makes a provocative comparison:
As large as Iraq looms on the scene today, as politically significant as the war is today, and as much as it will sour the public on intervention in the near future, I think we may be surprised at how quickly the effects of the war pass away and recede into the distance. Calamitous and awful as it has been, it still remains a war on a relatively limited scale and will wind up having a primarily regional impact. It has acquired the prominence that it has because it involves the superpower, but it will ultimately probably possess the historical significance of the Boer War or some other colonial misadventure of the British Empire.
One might also conjure up an analogy from America's own past - our long counter-insurgency in the Phillipines, which dragged on for more than a decade and cost more than 4,000 American and hundred of thousands (!) of Filipino lives, only to be completely forgotten by most people a century later. I've always been partial to the Filipino analogy, but it's worth remembering that the Phillipines, like the Transvaal, was a distinctly peripheral theater in the early 1900s, which substantially reduced the war's ripple effect on geopolitics; Iraq, on the other hand, is rather more centrally located, and sits athwart a region that matters a great deal to the global order (Edward Luttwak's provocations aside), at least until its oil wells run dry. So there's always a chance - albeit a small one, I think - that the Iraq War will prove a prelude to a larger conflagration of some kind, playing the Spanish Civil War to a Mesopotamian World War II.
If no such wider conflagration ensues, though, the best analogy for Iraq may be the one everyone always falls back on already, Vietnam - a war whose geopolitical significance proved negligible in the long run, and whose most profound consequences played themselves out on the American home front.
Oh, and just as Vietnam didn't mean the end of America's status as a global superpower, neither will the Iraq War. That outcome, at least, I'd be happy to put money on.