Well I haven't thought that seriously about it,but generally my opinion is they stay.

"You broke it,you fix it'

Now I'm not sure if they were right to enter in the first place.Maybe they were,maybe they weren't. But they did, and that means an obligation to get it on it's feet. Leaving now would have terrible consequences-civil war in an instant. And like you said, probably some kind of terroist state.

The American losses are bad, but compare to every other war it's still nothing. They've lost 3,000-4,000 in 6 years? As opposed to 406,000, military and civilian, in 4 years of WW2. While I sympathise with their families, no one forced them to join the military. You join, you know that there is the possiblilty you will die in a combat zone one day.It is the purpose of the military to fight.

So,I don't know how or when,but they need to get it resembling a functioning country.