He really has little choice.
And, alas, there's an obvious (but not very humanitarian) solution in Iraq. Get out of the way, and like the Shiites massacre the Sunnis. That'll settle things down, and quick, too.
If you mean "cut and run, let the Iraqis sort it out" --- no, if we do that, then Iran will be over the border to help their fellow Shia shortly after we leave.
Just upping and leaving definitely won't settle tensions in Iraq or in the region.