No More Troops
Daniel Benjamin and Michele Flournoy point out that we can't send any more troops to Iraq because there aren't any available. There are, of course, literally some additional soldiers hanging around who could be mobilized in a crisis, but at the moment all of the Army's combat strength is either deployed abroad or else somewhere in the reconstitution phase. This is also the difficulty with the retrospective version of the "more troops" theory which holds the occupation force should have been much larger in the beginning. An operation tempo of that scale wouldn't have been sustainable. Indeed, even the current tempo isn't genuinely sustainable -- it's causing recruiting problems and deteriorating troop quality while seriously degrading the amount of available equipment.
I note that Fred Kaplan pointed this all out in June 2005, but nobody seems to have paid attention. Meanwhile, it's unclear to me that more regular Army troops would do very much good. If you look at it, it's our Special Operations Forces units who've really been able to make a difference but, naturally, such units are in rather short supply. Had we concentrated our energies on a single counterinsurgency campaign in Afghanistan -- a context where we enjoyed more local, regional, and global legitimacy and were able to get some non-trivial assistance from allies -- then we might well have had the manpower necessary to pull off a good one. But this stuff turns out to be genuinely difficult, whether in the aftermath of a well-founded war (i.e., Afghanistan) or an ill-advised one (i.e., Iraq) and it was really the hight of hubris to think we could just move on to a second mission while an important one was still incomplete.