The CIA and its partners were never willing to give the opposition the weapons — especially the shoulder-fired antiaircraft missiles — that could have won the battle. The agency did provide anti-tank weapons that were potent enough that Assad was rocked in the summer of 2015, and analysts began to worry about “catastrophic success,” with the regime collapsing and jihadists filling a power vacuum in Damascus. Soon after that, Russia intervened.

The CIA’s biggest problem was that its allies couldn’t stop the dominance of al-Qaeda’s Syrian affiliate, Jabhat al-Nusra. The “vetted” opposition groups might pretend otherwise, but they were fighting alongside Jabhat al-Nusra, which rebranded itself this year as Jabhat Fatah al-Sham. The extremists attracted the other opposition groups for a simple reason: Their fighters were the most willing to die for the cause.

The United States tried to straddle this problem. In 2014, I visited the leaders of one of the vetted groups, known as Harakat al-Hazm, at a safe house along the Syrian-Turkish border. The fighters were despondent. The United States had just bombed a Jabhat al-Nusra camp nearby, seeking to kill militants from its so-called Khorasan Group. The CIA-backed fighters said this action had destroyed their credibility. They were right. Jabhat al-Nusra soon chased them from their headquarters.