“What we were looking for was an Iraq that was secure, stable and self-reliant, and that’s what we got here, so there’s no question that was a success,” said President Obama’s then-deputy national security adviser, Denis McDonough, in describing an October 2011 decision to station no U.S. forces in the country after that year.

It’s now obvious thatMcDonough spoke prematurely. Twenty-one months later, Iraq is suffering through the worst wave of violence since U.S. troops surged to stop a sectarian civil war in 2007 - and the United States’ ability to help is limited because of Obama’s decision to withdraw all U.S. forces.

Iraq’s renewed conflict gained attention recently because of the spectacular attack by al-Qaida on two prison facilities, including the Abu Ghraib facility near Baghdad. The coordinated assault led to the escape of hundreds of security prisoners, including a number of top al-Qaida leaders. Their release seems sure to fuel further violence not just in Iraq but also in neighboring Syria, where al-Qaida is waging war against the government of Bashar al-Assad as well as against more moderate rebel groups.

Bloodshed in Iraq already had been escalating steeply. During the past four months, nearly 3,000 people have been killed and more than 7,000 wounded, according to the United Nations.

Most were civilians victimized in al-Qaida bombings or attacks by the Sunni and Shiite militias that have been returning to action in Baghdad and the north of the country. All these forces have been energized in part by the Syrian war, which also pits Sunnis against Shiites and the related Alawi sect.

But Iraq’s troubles are also due to the narrowly sectarian and quasi-authoritarian policies of Prime Minister Nouri al-Maliki, who prosecuted Sunni leaders in his own government and sent troops to attack a Sunni protest encampment.

Maliki has recently taken small steps toward easing the country’s sectarian rifts, opening negotiations with Kurdish authorities who control an autonomous area in northern Iraq and allowing delayed provincial elections in Sunni-populated provinces to go forward.

However, much more decisive measures will be needed to prevent the country from sliding back into civil war. Despite the troop withdrawal, the United States retains some leverage, and now is the time to use it.

The Obama administration has for too long offered nearly unqualified support to Maliki. Now it should tell him that continued U.S. military assistance, including the promised delivery of major weapons systems, will depend on whether the government drops legal charges against Sunni leaders and reaches agreement with Kurds on long-outstanding territorial and revenue-sharing disputes. The White House should also insist that Maliki act to rein in Shiite militia groups.

At the same time, the White House should review whether it can offer Iraq’s armed forces additional support - in intelligence, training or materiel - to meet the growing threat from al-Qaida. If they are not countered, Iraq’s extremist forces will sooner or later become a U.S. security problem.