Backed by Russia and Iran, Mr. Assad has vowed to recapture all of Syria, including Raqqa and the areas beyond it where the Kurds have established a semiautonomous zone.

It remains unclear how far the United States would go to stop him.

Pentagon officials say that, for now, the American military will continue to defend areas like Raqqa, which American-backed forces reclaimed from the Islamic State. In June, the military shot down two Syrian drones that American officials said were threatening American-backed troops.

That posture has not changed, the officials said, and Syria experts say they expect it to continue for the next few months. What happens after that — and how willing the United States is to become engaged in a war against the Assad government and its international backers — is an open question.

“The issue of self-defense will certainly continue as long as the fight against ISIS continues,” said Andrew J. Tabler, a Syria expert with the Washington Institute for Near East Policy. “But what goes on after that with the political process, I don’t know.”

Trump administration officials acknowledge privately that the military campaign in Syria has by far outstripped the diplomatic campaign, to the point now where there is no real plan for what to do in a post-Islamic State Syria.

That is not for lack of trying by John Kerry, the secretary of state under President Barack Obama who sought a political solution for a post-Islamic State Syria, and Brett H. McGurk, the Trump administration’s point man on Syria.