At the time of the withdrawal, U.S. officials believed Iraq would manage any threats. Colin Kahl, a former senior defense official handling Mideast affairs, served his final day in office in Baghdad in December 2011 watching the last U.S. troops depart Iraq. According to Kahl, the administration believed that the Iraqi security forces could counter al Qaeda, and that the Sunni population had given up its insurgency. The administration realized there was a long-term risk that either a failure to politically accommodate the Sunnis or provocative behavior by the Iraqi security forces could revive Iraq’s Sunni armed resistance. But that was distant. What they didn’t foresee was the rise of al Qaeda in Syria and the prolonged sectarian war next door to Iraq. “No one could have anticipated the battlespace of Syria would merge with western Iraq,” Kahl said.

Through early summer 2012, when I spoke to officials inside the White House, they described the state of Iraq as “Iraqi good enough”—there could be violence and even authoritarian behavior by the government and abuses by the security forces; but as long as those excesses stayed within a certain level, it was tolerable. Iraq had always been an unstable and violent country with cruel leaders, so if its troubles didn’t spiral out of control, Iraq could muddle through.

In the meantime, the transition from a joint Pentagon-State Department venture in Iraq had been costly. Iraqi officials regularly complained that the Americans were absent. One senior government official warned me last February that the state was unraveling. The Americans had forged a consensus between the Shiites, Kurds and Sunnis—and now they had walked away from the bargain, this official said.