Still, some of the most informed voices in both parties said military action against the extremists in Iraq was necessary and probably belated.

“This is a very serious situation,” said Representative C. A. (Dutch) Ruppersberger of Maryland, the ranking Democrat on the House Intelligence Committee, reflecting the opinions of more hawkish Democrats. “This ISIL is dangerous. They’re strong. They have command and control. They have weaponry.” ISIL is an alternative name for ISIS.

That view is shared by Representative Adam Kinzinger, an Illinois Republican who said he backed the airstrikes — which he had been urging since January — but was concerned that Mr. Obama was not mounting a comprehensive enough effort to annihilate the threat from ISIS.

“If this is the beginning of a real effort to push back ISIS and destroy them, then I definitely support that,” Mr. Kinzinger, an Air Force veteran and Air National Guardsman who sits on the Foreign Affairs Committee, said in an interview. “Unfortunately, he did not have the intensity to come out and say that we have to destroy them. I think the president is frightened of re-engaging in Iraq, and I don’t think he really knows how to sell the reality of re-engaging to the American people.”

Senator Richard Blumenthal, Democrat of Connecticut, also said that Mr. Obama must do more to explain the goal and limits of the operation, but he spoke from a much different perspective.

“Humanitarian relief is necessary to prevent genocide and provide food and water to meet an urgent emergency, but the president owes the American people a better, fuller explanation of the scope and strategy of military actions,” he said. “I am deeply concerned that these actions could lead to prolonged direct military involvement, which I would strongly oppose.”

Antiwar groups were deeply critical of the airstrikes.

Angela Canterbury, the executive director of the Council for a Livable World and the Center for Arms Control and Non-Proliferation, said the groups “categorically condemn President Obama’s renewed military engagement by the United States in Iraq,” adding, “We cannot afford another unwinnable war overseas.”