Foes of the United States have cheered the prospect of an American withdrawal. But America’s regional allies, including Israel, Saudi Arabia and its partners in Syria, dread it.

They argue that American forces are still needed to provide a check on Russia, which considers Syria its strategic foothold in the Middle East, and Iran, whose proxies are building a military infrastructure in Syria to counter Israel.

A withdrawal could also leave the door open for the return of the Islamic State in some parts of Syria, the very reason the United States gave for intervening in the country to begin with.

The bombing that Mr. Trump was responding to was part of a battle between rebels and the Syrian government, one that the United States had withdrawn from long ago, in a part of the country where the United States has neither a military presence nor any clear allies.