In Washington, the Pentagon announced three separate strikes by multiple aircraft against militant positions it said were firing on Kurdish forces protecting Irbil, saying that they had "successfully eliminated" artillery, a mortar position, and a convoy of extremist fighters. Kurdish media and officials, who said the attacks had had a "devastating" impact on militant positions, claimed other, unconfirmed attacks that were farther afield.
U.S. officials also stressed that the American intervention was narrowly aimed at the protection of American diplomats and officials living in Irbil, where the large U.S. consulate has been swelled by evacuees from the embassy in Baghdad and the U.S. military runs a joint operations center alongside Kurdish forces.
"There are American military and diplomatic personnel in Irbil," White House spokesman Josh Earnest told a news briefing in Washington. "The protection of American personnel in Iraq is a top priority and one that merits the use of military force."
He emphasized that the authorization for air strikes "is very limited in scope," but did not rule out that there may be additional strikes to protect some of the tens of thousands of members of the minority Yazidi faith trapped by Islamic State fighters on a mountaintop.
The government of current Prime Minister Nouri al-Maliki requested the U.S. intervention, Earnest said. But he and other U.S. officials made clear that more comprehensive U.S. engagement in the battle against the militants will not happen unless feuding politicians in Baghdad establish a more inclusive government capable of resolving Sunni grievances that facilitated the Islamic States' rapid expansion.
The Iraqi parliament is scheduled to choose a new prime minister, perhaps as early as Sunday, according to U.S. officials who have made clear their preference for Maliki to stand down.
The first of the airstrikes came in the early afternoon - at dawn Washington time - and were carried out by two F/A-18 combat jets flying from the aircraft carrier USS George H.W. Bush in the Persian Gulf. The aircraft dropped 500-pound laser-guided bombs on a mobile artillery piece that had been used to shell Kurdish positions, the Pentagon said. The strike occurred in Makhmour, a town southwest of Irbil, according to Mahmood Haji, an official at the Kurdish Interior Ministry.
Two more announced strikes came in late afternoon, Iraq time. An MQ-1 Predator drone armed with Hellfire missiles struck an Islamic State mortar position. When fighters returned to the site moments later, "the terrorists were attacked again and successfully eliminated," according to Pentagon spokesman Rear Adm. John Kirby.
Less than an hour later, four aircraft dropped eight laser guided bombs on a seven-vehicle convoy and a mortar position nearby, the Pentagon said. Those strikes took place near the Khazer checkpoint on the road between Mosul and Irbil, according to Haji, the Kurdish official.
As news of the strikes spread, jihadist fighters and supporters took to Twitter to express glee that the United States had become embroiled in their battle, threatening to shoot down planes, exact revenge and conquer other American allies elsewhere in the region.
"This crisis will become a gift and you shall remember this: Our State will enter Irbil and America will fall, and then the Gulf will be ours," one purported jihadi tweeted.
"Your announcements do not scare us; the biggest enemy of the State is the Americans," said another.
Obama authorized the strikes in response to a powerful Islamic State offensive launched a week ago across northern Iraq, in which towns and villages occupied mainly by members of Iraq's ancient Christian minority, as well as the Yazidis, have been overrun.
At the same time, he dispatched U.S. military aircraft to drop food and water to the besieged Yazidis, who fled the town of Sinjar to a nearby mountain to escape the advancing militants. Obama said airstrikes might also be used to break the militant siege of the mountaintop, if Kurdish forces are unable to do so.
After weeks of appealing to the United States for arms and ammunition to help in the fight against militants, Kurdish officials expressed gratitude for the intervention.
"We never lost hope that our friends would come when the circumstances were there," former Iraqi foreign minister Hoshyar Zebari told a news conference in Irbil.
He said the strikes had made a significant impact on the ground and would enable Kurdish forces to regain the ground they have lost to the extremists in the past week.
"Our intelligence ... is that it has been a devastating blow" to the militants, Zebari said. "Commanders have seen dramatic changes on the front lines. ... We have already seen some withdrawals."
Whether strikes limited to the northeastern edge of the vast territory controlled by the Islamic State will impact its control elsewhere is in question, however.
Response to the strikes was limited from the U.S. Congress, which is in the midst of a late-summer recess. While most who offered an opinion supported Obama's decision, some Republicans criticized him for waiting too long as Islamic State forces spread across Iraq over the last two months.
Others warned Obama against expanding the effort without seeking congressional approval.
"If further sustained military action is necessary, it is incumbent on Congress to review all the facts, debate the issue and vote to authorize any additional sustained military action," Rep. Bobby Scott, D-Va., said in a statement.
Three main factors motivated Obama's decision to authorize the airstrikes and humanitarian airdrops, Earnest said: the "deeply disturbing" situation at Mount Sinjar, reports that Islamic State insurgents were advancing toward Irbil and the progress that Iraqi politicians have made in forming a new government after months of stalemate in the wake of parliamentary elections.
He said he was "not in a position to offer a specific date" on when the campaign would end, but he reiterated Obama's pledge that "the United States will not be dragged back into a prolonged military conflict in Iraq" and that the intervention would not include ground troops.