Removing U.S. troops from Iraq.
Reading the various accusations that the Obama administration has surrendered Iraq to Iran, I think it's worth keeping this in mind:
By nature of religious affinity and geography, Iran was always bound to play an outsized role inside Iraq. As several people have pointed out already, the era when Iranian influence was at its lowest ebb inside Iraq was when a Sunni autocrat ruled the country. Moreover, in a democratic Iraq with a loser political structure, Iran is going to have far more levers of influence inside Iraq. It's unavoidable.
This underscores, I think, a lot of the naivete that drives the "stay in Iraq" policy advocacy. Follow the chain: the U.S. has to knock off a Sunni Arab dictator, and has to install a democratic government in its wake, and has to install a democratic government that is friendly to Washington's strategic priorities, and has to create a political system immune from too much influence from its neighbor, and has to commit tens of thousands of troops and billions of dollars to the effort indefinitely irrespective of America's balance sheet.
It's also worth stating the blindingly obvious: Iran increased its influence inside Iraq under the nose of roughly 50,000 U.S. troops from 2008 on (and over 150,000 before then). The U.S. may have "beaten back" some Iran-affiliated militias during and after the Surge, but it never "defeated" them. Keeping 20,000 to 30,000 U.S. troops in the country to chase Iranian agents and their Iraqi sympathizers around southern Iraq is hardly going to do the trick, and in an era of tighter resources, it's a rather decadent waste of time and money. I do think a Lebanon-style civil war, with Iranian-funded Shiite militias battling Saudi-funded Sunni militia (and Turkey bombing the Kurds in Northern Iraq) is a distinct possibility, which is why the removal of U.S. troops is ultimately a wise choice.