Administration officials said the United States did not seek an endorsement of military action from the Arab League. It sought condemnation of the use of chemical weapons and a clear assignment of responsibility for the attack to the Assad government, both of which the officials said they were satisfied they got.

The Obama administration has declined to spell out the legal justification that the president would use in ordering a strike, beyond saying that the large-scale use of chemical weapons violates international norms. But officials said he could draw on a range of treaties and statutes, from the Geneva Conventions to the Chemical Weapons Convention.

Mr. Obama, they said, could also cite the need to protect a vulnerable population, as his Democratic predecessor, Bill Clinton, did in ordering NATO’s 78-day air campaign on Kosovo in 1999. Or he could invoke the “responsibility to protect” principle, cited by some officials to justify the American-led bombing campaign in Libya.

“There is no doubt here that chemical weapons were used on a massive scale on Aug. 21 outside of Damascus,” said the White House spokesman, Jay Carney. “There is also very little doubt, and should be no doubt for anyone who approaches this logically, that the Syrian regime is responsible for the use of chemical weapons on Aug. 21 outside of Damascus.”

A number of nations in Europe and the Middle East, along with several humanitarian organizations, have joined the United States in that assessment. But with the specter of the faulty intelligence assessments before the Iraq war still hanging over American decision making, and with polls showing that only a small fraction of the American public supports military intervention in Syria, some officials in Washington said there needs to be some kind of a public presentation making the case for war.