During a key battle in the rugged mountains of a northern province earlier this month, U.S.-backed Syrian rebels collapsed before an assault by al-Qaida fighters. Some surrendered their weapons. Others outright defected to the militants.

A detailed account of the battle in Idlib, from a series of interviews with opposition activists by The Associated Press, underscores how the moderate rebels that Washington is trying to boost to fight the Islamic State group are instead hemorrhaging on multiple fronts.

They face an escalated assault by Islamic extremists, which activists say are increasingly working together to eliminate them. At the same time, a string of assassinations has targeted some of their most powerful commanders. […]