BEIRUT — The Obama administration’s Syria strategy suffered a major setback Sunday after fighters linked to al-Qaeda routed U.S.-backed rebels from their main northern strongholds, capturing significant quantities of weaponry, triggering widespread defections and ending hopes that Washington will readily find Syrian partners in its war against the Islamic State. Moderate rebels who had been armed and trained by the United States either surrendered or defected to the extremists as the Jabhat al-Nusra group, affiliated with al-Qaeda, swept through the towns and villages the moderates controlled in the northern province of Idlib, in what appeared to be a concerted push...