Many warned that if weakening ISIS strengthened Mr. Assad and was not accompanied by political enfranchisement of the Sunni majority in Syria, the strikes could backfire, driving more Sunnis to support or tolerate ISIS. Others worried that Syrian civilians could be killed in the attacks.

Image President Bashar al-Assad of Syria, right, spoke with the United Nations envoy to Syria, Staffan de Mistura, in Damascus on Thursday. Credit... SANA, via Associated Press

“Some parts of Syria haven’t been destroyed yet, but after the strike nothing will be left,” said Matar, a spokesman for Ahrar al-Sham, an ultraconservative Islamist group that opposes ISIS, giving only his first name for security reasons. “Our war is with Bashar,” he added, complaining that America had not responded to Mr. Assad’s indiscriminate use of shrapnel-filled barrel bombs. “Why did the Americans wait until now to strike?”

And even those most supportive of the strikes complained that America had abetted the rise of extremists like ISIS by failing to aid other insurgents earlier, and was only attacking it now that it had swollen to pose a threat to America and the world.

The reactions underscored the conundrum the United States faces in wading into the Syrian battlefield. Until now, Mr. Obama has strenuously resisted pleas from Syrian insurgents for more help in fighting Mr. Assad’s crackdown against an insurgency that began with political protests, citing concerns that arms given to insurgents would end up in extremist hands.

Now, framing the attack on ISIS as driven by American national security concerns, Mr. Obama faces mistrust from both sides. Many insurgents consider his action too little too late, and say his declaration that America stands with “people who fight for their own freedom” rings hollow. And the Syrian government and its supporters express suspicion that the strikes could be a cover to attack the government.