There's a corollary to our manpower problems: if you look at the numbers you see that we basically went into Iraq with roughly two thirds of the sustainable manpower we could have applied. So the "Rumsfeld Doctrine" of light, highly mobile force, while it might be honestly held, is at least convenient in that we didn't have any choice in the matter.
Or, rather, we did. We could have worked to assemble a genuine coalition (which meant taking a year at least to give sanctions time to "work", which of course they already had) or we could have increased manpower by instituting conscription (at about an equal cost in time). Whatever we actually believed about WMDs, we knew for certain that Iraq's delivery capabilities were next to nil.
The obvious point about option 2 is that a draft is a political faultline that's been building up pressure for thirty years. And option 1 is dismissed as turning our sovereignty over to the blue helmets. Those are political considerations, not military ones. And it is certainly fair to say that things went FUBAR, and Americans died because of it. In the end it's funny that our Steely-Eyed Rocket Man with the firm vision of Freedom dare not make decisions without an eye to the polls. But nobody's laughing.