We can't maintain troop levels after the Spring, so the beginning of withdrawal is inevitable.
A fast withdrawal would be disastrous, so we can't do that.
We have to have some US presence just in case.
As Petraeus told her, it would be horrible to just sit in bases doing nothing while civil war raged.
In other words, we should withdraw troops down to the level at which we wouldn't be able to do much other than guard our own stuff, which would be horrible. Or something. It's all utterly incoherent.