Basically the same thing that will happen if we wait. The Iraqis will decide who runs Iraq. Iran as a large, wealthy and well armed neighbor, will exert some influence in favor of their fellow Shiites. The Turks will get nervous about the autonomous Kurds. The Sunnis will try to enlist money and support from fellow Sunnis. Syria, as a puny, weak neighbor will have less influence. Some Iraqis will kill other Iraqis.
Juniorbush and Cheney will bitch and moan about how whoever caused the "early withdrawal" robbed them of certain victory.
Drudge, Rush and Hannity will agree with them will agree.
Several people will write books and appear on talk shows a lot.