Military manpower is a serious structural constraint on the Bush administration.

washingtonpost.com: Army Having Difficulty Meeting Goals In Recruiting.
With force levels in Iraq projected to be at least 120,000 U.S. troops the Army will be stretched very, very thin. While the idea of a draft is political poison, there is no question that steps will be needed to improve recruitment. Recruitment bonuses have already been increased dramatically, and they will probably have to go higher. Reserve officials worry that not only are recruitment falling short, but that qualitative sacrifices are being made as well. If the Army Guard and Reserve cannot retain top quality leaders, there will be further personnel losses on top of those already resulting from the dangerous conditions of service in Iraq and the heavy economic costs suffered by many citizen soldiers. More than the deadly effectiveness of the Iraqi insurgents, these structural considerations will keenly affect U.S. decisionmaking. Don't expect the generals of Donald Rumsfeld to talk too openly about this, but you can bet it is a number one topic of concern in the Pentagon.