The announcements reflected the deepest American engagement in Iraq since U.S. troops withdrew in late 2011 after nearly a decade of war. Obama, who made his remarks in a steady and somber tone, has staked much of his legacy as president on ending what he has called the "dumb war" in Iraq.
Obama said the humanitarian airdrops were made at the request of the Iraqi government. The food and water supplies were delivered to the tens of thousands of Yazidis trapped on a mountain without food and water. The Yazidis, who follow an ancient religion with ties to Zoroastrianism, fled their homes after the Islamic State group issued an ultimatum to convert to Islam, pay a religious fine, flee their homes or face death.
Mindful of the public's aversion to another lengthy war, Obama acknowledged that the prospect of a new round of U.S. military action would be a cause for concern among many Americans. He vowed anew not to put American combat troops back on the ground in Iraq and said there was no U.S. military solution to the crisis.
"As commander in chief, I will not allow the United States to be dragged into fighting another war in Iraq," Obama said.
Even so, he outlined a rationale for airstrikes if the Islamic State militants advance on American troops in the northern city of Irbil and the U.S. consulate there in the Kurdish region of Iraq. The troops were sent to Iraq earlier this year as part of the White House response to the extremist group's swift movement across the border with Syria and into Iraq.
"When the lives of American citizens are at risk, we will take action," Obama said. "That's my responsibility as commander in chief."
He said he had also authorized the use of targeted military strikes if necessary to help the Iraqi security forces protect civilians.
Obama spoke following a day of urgent discussions with his national security team. He addressed the nation only after the American military aircraft delivering food and water to the Iraqis had safely left the drop site in northern Iraq.
The Pentagon said the airdrops were performed by one C-17 and two C-130 cargo aircraft that together delivered a total of 72 bundles of food and water. They were escorted by two F/A-18 fighters from an undisclosed air base in the region.
The planes delivered 5,300 gallons of fresh drinking water and 8,000 pre-packaged meals and were over the drop area for less than 15 minutes at a low altitude.
The president cast the mission to assist the Yazidis as part of the American mandate to assist around the world when the U.S. has the unique capabilities to help avert a massacre.
In those cases, Obama said, "we can act carefully and responsibly to prevent a potential act of genocide."
Officials said the U.S. was prepared to undertake additional humanitarian airdrops if necessary, though they did not say how quickly those missions could occur.
Administration officials said they believe unilateral U.S. strikes would be consistent with international law in part because the Iraqi government has asked for Washington to take military action. They also said Obama had the constitutional authority to act on his own in order to protect American citizens.
Still, there was no guarantee that the president's threat of military strikes would actually be followed by action. He similarly authorized strikes in Syria last summer after chemical weapons were deployed, but those attacks were never carried out, in part because of domestic political concerns and also because an international agreement to strip Syria of its stockpiles of the deadly gases.
The president has also faced persistent calls to take military action in Syria on humanitarian grounds, given that more than 170,000 people have been killed there.
Critics, including some Republicans in Congress, have argued that Obama's cautious approach to Syria has allowed the Islamic State group to flourish there, growing strong enough to move across the border with Iraq and make swift gains.
Republican Sens. John McCain of Arizona and Lindsey Graham of South Carolina praised Obama's proposed actions Thursday night but said much more will be necessary.
"This should include the provision of military and other assistance to our Kurdish, Iraqi, and Syrian partners" who are fighting the militants, airstrikes against the militants' leaders and forces and support for Sunni Iraqis who seek to resist the extremists, they said in a statement.
In light of the militants' advances, Obama dispatched about 800 U.S. forces to Iraq earlier this year, with those troops largely split between joint operation centers in Baghdad and Irbil.
More than half are providing security for the embassy and U.S. personnel. American service members also are involved in improving U.S. intelligence, providing security cooperation and conducting assessments of Iraqi capabilities.
Officials said there were no plans to evacuate those Americans from Iraq but that the U.S. was conducting enhanced intelligence flights over Irbil with both manned and unmanned aircrafts in order to monitor the deteriorating conditions.
If the president were to order actual airstrikes in Iraq, it's all but certain he would proceed without formal congressional approval. Lawmakers left town last week for a five-week recess, and there was no sign that Congress was being called back.
However, officials said the White House was in contact throughout Thursday with some lawmakers, including House Speaker John Boehner, R-Ohio, and Senate Minority Leader Mitch McConnell, R-Ky.
Some Republicans have expressly called for the president to take action and have said he doesn't need the approval of lawmakers.
Associated Press writers Josh Lederman and Darlene Superville contributed to this report.
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