The Islamic State is now reeling in Syria. It has been battered by strikes from a host of enemies, from the United States and its regional allies to the Syrian government that is backed by Russia and Iran. It no longer holds one-third of the country, according to American officials who say that the group has lost around half of the territory it once controlled.

In past years, the Pentagon and its allies could stay out of the Syrian government’s way — and that of Mr. Assad’s backers in Russia and Iran — as all fought the Islamic State. Now, all sides are converging on a smaller piece of territory, resulting in competing forces increasingly turning on one another, in addition to the common enemy.

Captain Davis, at the Pentagon, noted that when American-backed ground troops are confronted by “armed drones, that leaves us with no choice but to defend ourselves and our partners.”

He said that the downing of an Iranian-made drone this week was done in self-defense. Defense officials insist that does not amount to a greater United States involvement in the broader war.

But privately, American military officials acknowledge that they are quickly running out of space in Syria to stay out of Mr. Assad’s way — not to mention Russia’s and Iran’s.

In Europe, the new president of France, Emmanuel Macron, announced that he would be taking a distinctly different tack on Syria than his predecessor. Mr. Macron said that getting rid of Mr. Assad was no longer a top priority.

Instead, Mr. Macron said, getting rid of terrorists is more important — and he is prepared to work with anyone toward that end, including Moscow.