Well, wouldn't it kind of help to be pretty stupid (or desperate for paying work and 3 meals a day)? What I mean is, high intelligence would seem to militate strongly against volunteering to join a military that is likely to send you to a mess like Iraq, wouldn't it?

(with some exceptions...I'm sure there are intelligent people who imagine they're doing their patriotic duty over there...after all, the Third Reich found plenty of those, didn't it? They did start really dwindling after about '43, though...eventually people catch on and aren't so easily fooled any longer.)

The only thing that sobers up a great power that is drunk on its own rhetoric and conquest is a major military defeat. The USA has been careful to avoid attacking anyone lately who can possibly inflict such a defeat on them. But even an ignominious withdrawal...as from Vietnam...can also be pretty chastening. It causes the hawks to lick their wounds and retire from the scene for a little while. Like...5 or 10 years at best, usually.