A week or so ago, Matthew Yglesias made a really good point. One commonly cited reason why the US needed to stay in Iraq and stabilize the country was that if we abandoned it, it would cause a regional war. Saudi Arabia would intervene on the side of Sunnis, Iran on the side of Shias, boom WW3.
However, in the last few weeks we've seen the development of some escalations in violence in Yemen which appears to be the result of some proxy wars going on. You have the governments of Saudi Arabia and Yemen attacking Shia rebels in northern Yemen, which in turn are being backed by Iran.
This proxy war is taking place and yet, as Matt pointed out, nobody is saying that we should be rushing 100,000 US troops into the buffer to prevent a war between the Saudis and Iran. This isn't to say that I don't think Iraq isn't important for other reasons. But purely as a means to prevent a regional war, this parallel situation suggests maybe that isn't a good use for US troops.