But Mr. Obama is facing complications as he works to assemble support at home and a coalition abroad. The Turkish government, for example, has been fearful of an aggressive retaliation against ISIS because of concerns that the group would harm the 49 Turkish citizens it is holding, including its consul general, after ISIS raided the Turkish Consulate in the Iraqi city of Mosul.

On Tuesday, Defense Secretary Chuck Hagel met with Turkish leaders to discuss their contribution to the effort, but emerged with no concrete commitments. A Turkish official expressed concerns that weapons sent to Syrian rebels to fight ISIS could end up in the hands of Kurdish fighters whom Turkey regards as a terrorist group.

The White House is sending Secretary of State John Kerry to Saudi Arabia this week to enlist the Saudis, who have been a vital source of funding to groups opposing the Assad government. Saudi Arabia, while supportive of the United States, worries that going to war with ISIS could provoke a backlash among Sunni extremists in its own population.

At the dinner on Monday, several participants said, Mr. Obama expressed confidence that a robust coalition would coalesce against ISIS over time. They said he presented a comprehensive plan that included military, diplomatic and ideological components, based on trying to counter the story line that ISIS has propagated in the Arab world.

“The big picture is that this is going to be a very long-term proposition, that American leadership is necessary, and that this can’t turn into a U.S. versus Sunni battle,” said one of the guests, Samuel R. Berger, who was national security adviser to President Bill Clinton. “It has to be us helping the Sunnis battle the Sunni extremists.”

Another participant, Strobe Talbott, a former deputy secretary of state who is the president of the Brookings Institution, said, “He regards ISIS as a new phenomenon that requires not thinking about it as ‘some of them here, some of them there,’ but ‘wherever it is, to go after it.’ ”

Jane Harman, the president of the Woodrow Wilson Center, said that while the Assad government was a major topic of discussion, she and other participants told Mr. Obama that he could order military action in Syria without fear of helping Mr. Assad, since ISIS was occupying ungoverned territory that his forces were unlikely to reconquer.