In the weeks since President Trump unexpectedly announced that American troops would withdraw from Syria — “and they’re coming back now” — rejecting the counsel of generals and advisers, some administration officials stepped down and others tried to chart a path forward.

If anyone had any illusions about just how complicated the withdrawal might be, they were unlikely to retain them after the latest bad news out of the war-torn country.

On Wednesday, Americans troops were among the dead after a bombing attack in northern Syria that was claimed by the Islamic State. They were the latest deaths in a seemingly intractable conflict that began with a popular uprising in 2011 and grew into a complex entanglement of multiple foreign powers.

President Barack Obama sent American troops to Syria in 2015 as part of a coalition against the Islamic State, or ISIS. But in announcing the withdrawal in December, Mr. Trump declared that “historic victories” over the militants meant the time had come for American forces to come home.