K., 28, is on the line from Aleppo. He speaks to Al-Monitor via Skype, with a soundtrack of barrel bombs and gunfire, but that's no news — and not the source of the disappointment and dismay that mark his voice. A trainee doctor turned sniper, he was long one of about 5,000 men that the Islamic State of Iraq and as-Sham (ISIS), the al-Qaeda offspring currently in the spotlight for its stunning gains in Iraq, is estimated to have deployed in Syria.

ISIS controls a large area stretching across the border and looks stronger than ever. I spent time in autumn 2013 embedded with ISIS fighters in Aleppo, K. among them. K. was initially seduced by the group’s fighting power and the values it sought to represent. He wasn't deeply religious, he says. He joined Islamist groups, fighting with Ahrar al-Sham and Jabhat al-Nusra before joining ISIS, because they were the only ones to provide real training and oppose rampant looting and crimes of any kind. "They had not only rules, but values."

But everything changed between December and January, when the regime of Syrian President Bashar al-Assad began a massive air campaign that has yet to end. "It was some kind of hailstorm. We had to rally together. We had to create a diversion — to attack Latakia, or Damascus, whatever, but all together. Instead, amid the smoke, the fire, the rubble, infighting erupted, and the struggle became one against infidels, rather than against Assad. While Aleppo was vanishing, we were hunting infidels house by house, or even better, ruin by ruin," he complained. With the help of a foot infection, he quit ISIS.

"Now I realize that they didn't come for Assad,” he says. "That's why they focused on eastern Syria. Our priority was Aleppo. We repeated that we had to advance from north to south — Aleppo, Hama, Homs, down to Damascus — and they replied that the priority was to deprive the regime of the bulk of its economy — of Raqqa and Deir Ez-Zor, which have oil and agriculture. But the economy here collapsed long ago. Assad is funded by Iran, not by oil — we were just cannon fodder. And not only is Assad no longer the lesser evil, he will soon become a US ally. You see? He already bombed Raqqa. Who remembers our demands anymore? Democracy, justice … Who remembers why we started this whole thing up?” His bitterness toward his former comrades is evident. “We have been defeated by ISIS, not by Assad.”