The American withdrawal was going to lead to a time of testing in Iraq in any case, but political overreaching by the Shiite majority is making a bad situation worse.During the first years of the American presence, it was the Sunnis who fatally misread the power realities of Iraq. Having been dominant in Iraq since Ottoman times, many Sunnis overestimated their demographic weight in Iraq and underestimated the ability of the once-despised Shiites to organize and hold power.That delusion was blasted away in waves of sectarian strife and ethnic cleansing; the Sunnis painfully woke up to some unpleasant realities and ultimately accepted that the days of Sunni domination in Iraq had come to an end.But lately it is the Shiites who seem to be overplaying their hand. The balance of power is changing in the Middle East in ways that strengthen Iraq’s Sunnis and weaken the Shiites. Iran is isolated, and its client in Syria is on the ropes. Turkey has reentered the region as a Sunni power, increasingly hostile both to Assad and to Iran.If (perhaps when) Assad falls, the balance of power in Iraq will shift. Bordered by Saudi Arabia, Jordan and Syria, Iraq’s Sunnis will have access to arms and supplies that the government cannot interdict. Turkey, deeply concerned about Iraq’s Kurdish region, will want a stronger hand in the country. The US, deeply concerned to bolster Iraq’s central government while we still had a military role in the country, will be less committed to the government — especially if it is seen as tilting toward Iran.Iraq — and Syria also, for that matter — could be facing an extended period of ethnic and religious strife. Iran, Saudi Arabia and Turkey all have interests in Iraq; all can find clients to support. Lebanon is the unhappy example of a Middle Eastern country where domestic factions supported by foreign patrons fought for years. As Assad falls, Turkey rises, and the Saudis and other Gulf powers intervene, the “Shi’a Crescent” that Sunnis once feared may become a “Chaos Crescent” from the Mediterranean to Iran.A slower US military withdrawal would have made this less likely; Americans are likely to regret leaving so precipitously (though once out of Iraq we should avoid going back in), and the Shiites may come to regret letting us leave.