Let's suppose, not unreasonably, that should we do so, Iraq devolves into a really fun faction fight.
Sadr vs. pro-Iran Shia vs. Ba'athist/Nationalist Sunnis vs. Al-Qaeda vs. Kurds vs. Mixed Moderates vs. blahblahblahblahblah.
Let's suppose, like in Afghanistan after the collapse of the Soviet-backed regime (five minutes after the last Russian left Afghanistan), the winner is not (shockingly enough) someone who is Into peace, love, fluffy bunnys, and the Will of the Iraqi People.
Let's take a look around the Middle East and make the easy bet that the winner will be someone who is really ruthless and amoral and willing to shove people into wood chippers if it scares enough of their relatives into shutting up. I mean, that describes pretty much the history of Iraq since the time of the Assyrians.
So, here's my question. We ignored Afghanistan and said to ourself, "Hey, if they want to murder and rape and pillage each other, it isn't our problem, is it?" And that lasted for about a decade, and then we had to Do Something About It.
So if we pull out next year, does that mean that in 2017, after I hit 18 years in the Army and build the Dream House and have four kids running around underfoot and am putting together my job search stuff to find a cushy civillian gig, instead of going ROAD (Retired On Active Duty) and picking up an AC/RC slot in Austin or ROTC advisor at UT-Austin (after sending my Branch Manager's civilian secretary flowers, chocolate, a pile of small unmarked bills, and Jack Daniels by the quart) I'm going to have to come back to this desolate hell-hole with a whole friggin' platoon (or, God Forbid, a company) to worry about?