Al-Qaida’s Syrian affiliate, the Nusra Front, which has long been viewed by that country’s rebels as an ally in the battle to topple President Bashar Assad, has turned on them in recent days, forcing the beleaguered rebels into a three-front war that they say they are sure to lose unless the United States changes policy and sends them more weapons.

Commanders warned Wednesday that assaults this week by Nusra could cause the collapse of rebel front lines, which already were under stress from fighting the Islamic State and the Assad government.

“We are defending our existence,” said Gen. Muhammad Hallak of the Syrian Revolutionary Front, one of the rebel groups. Without more assistance from the U.S.-led coalition battling the Islamic State, “we will withdraw our forces from the front with the Islamic State and the regime and work only to save ourselves.” […]