Marc Lynch speculates about why Bush has decided not to lower troop levels before the end of his presidency:
Bush clearly fears - rightly or wrongly - that a reduction of American military forces would lead to an outbreak of violence which might interfere with the McCain campaign's 'return on success'. The military commanders, meanwhile, understand all too well the fragility of the current situation in Iraq: the risks of the upcoming 'transition' of the Sons of Iraq program, the conflicts between Iraqi government forces and the Kurdish Peshmerga in Diyala, the intensifying conflicts between the Anbar Salvation Council and the Islamic Party in Anbar, the political stalemate which led to the postponement of the provincial elections, and so on. They don't want fewer troops at their disposal if they can get away with it because, frankly, why would they if they don't have to? Bush's refusal to significantly draw down troops as a "return on success" tells you what you need to know about how the decision makers really feel about the amount of "success."