“I will never go back,” one Iraqi Sunni vowed this week as he and his wife and children arrived in the U.S. to start a new life as refugees here. More ominously, a former cabinet minister of one of the Shiite-led governments that replaced Saddam spoke confidently in a private interview this year of finishing off the Shiite war against Sunnis when the American troops finally leave.

Experts say there are still some reasons to hope things don’t get that bad. But if they do, America—overstretched as it is in Afghanistan, overstretched as it is in its economy—will have little choice but let Iraqis hash it out on their own…

Even if Iraq falls apart, even if the Iraqi government appeals for help, “the cost of sending forces back in after we’ve packed up and left would be something,” Sanok said. “The cost of trying to go back in would be overwhelming.”