Specifically, the assumption that areas in Iraq from which U.S. forces withdraw will continue to see improvements to security. Kimberly Kagan writes today in the Wall Street Journal:
Coalition and Iraqi forces have not finished clearing Ninevah province, Salah ad-Din and parts of Babil. Major operations continue against al Qaeda remnants in Ninevah, Salah-ad-Din, Diyala, Kirkuk and Wasit provinces. Fighting between Iraqi Security Forces (aided by coalition special forces and our Georgian, Polish and British allies) and Mahdi Army militias continues in the south. The withdrawal to 15 brigades already assumes that these operations will be successful. It provides no cushion for unexpected developments or unforeseen enemy responses. There is thus no military basis at all at the present time to recommend additional reductions in 2008.
The irony here is that the greatest obstacle to consolidating the security gains of the last year may well be the security gains of the last year, as reduced levels of violence have only added to the pressure to reduce force levels. If the violence has subsided, the logic goes, we can begin bringing the troops home. But as Kagan says "We won more than we had hoped, and now we may need to defend it more than we had planned."