Peter Beinart:

[Thomas] Ricks[…] says the U.S. will need 30,000 to 50,000 troops in Iraq for a long time if it wants to avoid a civil war that drags in the entire region.

My guess is that Ricks’ view will prevail. The military has invested epic quantities of money and blood in Iraq, and U.S. commanders don’t want it to be in vain.

That’s when you’re down big.  And when you’re winning, you can’t leave in the middle of a hot streak.  All these very good arguments evaporate when you leave the table.