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A senior administration official said that the safety of U.S. military personnel based in Erbil is specifically what prompted Mr. Obama to authorize targeted airstrikes.

“What we’ve seen in recent days is further [Islamic State] advances that were beginning to threaten the periphery of Irbil,” the official said. adding that it’s a dynamic situation the U.S. will continue to monitor, the official said.

The Islamist group’s march through Iraq and its religious persecution of Christians and ethnic Kurds has left the nation on the brink, and the administration surprised and somewhat impressed by its speed.

“It was swift. It was effective,” a second administration official said. “They acted with tremendous military proficiency.”

Led by Abu Bakr al-Baghdadi and estimated to have as many as 10,000 fighters across Iraq and Syria, the Islamic State appears bent on imposing Islam’s Shariah law on Iraq through violence, though the group is not considered strong enough to successfully take Baghdad, home to a massive Shiite population and defended by Iraqi security forces.

Al-Baghdadi, U.S. officials have said, was thought to have been killed on at least three occasions. He was taken into U.S. military custody in February 2004 in Fallujah during the early days of the Sunni insurgency, after the successful invasion of Iraq in 2003.

In December 2004, however, he was released in what some former U.S. military officials said was a “failing of the procedures.”

A decade later, the Islamic State represents a grave threat to the Iraqi government and prompted an unprecedented level of cooperation among the U.S. military, the Iraqi central government and military, and local Kurdish authorities.

“We began to develop what was really a fairly historic level of cooperation between the Iraqi air force and the [Kurdish fighters] providing tactical air strikes on the ground,” the second administration official said.

The U.S. and international partners have urged all sides in Iraq to come together to form a new coalition government, having lost faith in Iraqi Prime Minister Nouri al-Maliki’s ability to govern effectively and maintain a stable and secure country.

But a transition government won’t materialize overnight, and the more immediate concern is the Islamic State group tearing its path through Iraq.

Mr. Obama’s actions may appease some critics who earlier in the day blasted the administration for possibly allowing an all-out genocide to be carried out by Islamic State militants.

In a letter to the president Thursday, Rep. Frank R. Wolf of Virginia accused the administration of ignoring genocidal atrocities taking place against Iraqi Christians and Yazidis, whose traditions encompass a range of Islamic and ancient Persian beliefs.

Mr. Wolfe reminded Mr. Obama of a 2012 speech at the U.S. Holocaust Memorial Museum where the president said a newly created Atrocities Prevention Board would make the deterrence of genocide and mass atrocities “a core national security interest and core moral responsibility.”

“Tragically, mass atrocities are happening again today — and on your watch,” Mr. Wolf wrote in the letter. “Genocide is taking place today in northern Iraq, where the Christian and [Yazidi] populations are being exterminated by [the Islamic State]. There is no question that systematic and targeted brutality is occurring.

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