‘Everyone Is Scared of ISIS.’

I’ve spent much of the last week in Antakya, an ancient city, known to Byzantine Christians as Antioch, which now serves as a bivouac for Syrian rebel fighters and a jumping-off point for journalists and humanitarian actors working in Syria, which lies 20 miles to the west. One subject preoccupies everyone in the Turkish town: not the brutality of the regime in Damascus, but the nihilistic violence of the foreign jihadi group known as the Islamic State of Iraq and al-Sham (ISIS).

Firas Tammim, a native of the Syrian city of Latakia who now brings medical supplies and other goods to the region, said to me, "I don’t want to say Assad is better, but at least he didn’t arrest or kill people because they were smoking." Tammim showed me a picture on his phone of a crowd of villagers, including children, witnessing an ISIS beheading of an alleged infidel. "Think what this does to these children," he said. Over time, Tammim said, Syrians are becoming inured to what they once would have found unspeakable.

ISIS appears to have up to 8,000 soldiers in Syria, a tiny number compared with the 100,000 or so rebel fighters. But the group’s medieval ideology, as well as its pathological obsession with enforcing Islamist rectitude in the towns and cities its soldiers have infiltrated, has made it a source of terror. One evening I was sitting at an outdoor cafe where a grizzled man was steadily smoking a hookah and shooting jets of tobacco smoke through his nostrils. He called himself Abu Abdul, and he was a fighter with a brigade affiliated with the Free Syrian Army (FSA), the "moderate" forces backed by the West. We talked about the jihadists. Then he said something else. "He asks that you not mention the name of his brigade," my interpreter said. "Everyone is scared of ISIS."

President Bashar al-Assad has received two enormous gifts in recent months. The first is the Russian-brokered deal to remove Syria’s chemical weapons, which distracted attention from his relentless campaign to kill and terrorize his enemies and also compelled Western governments to work with him as the country’s legitimate ruler. The second is ISIS, which has also deflected attention away from the war between the regime and the rebels and has vindicated as nothing else could Assad’s persistent claim that he is confronting, not political opponents, but "terrorists," as his foreign minister, Walid al-Muallem, recently claimed at the United Nations.

For this reason, it has become a fixed conviction in Antakya that ISIS functions as a secret arm of the regime. This sounds like an all-too-understandable conspiracy theory, yet even Western diplomats I’ve spoken to consider it plausible, if scarcely proved. In the summer of 2012, Assad released from prison a number of jihadists who had fought with al Qaeda in Iraq and who are thought to have helped formed ISIS. Reporters, activists, and fighters also note that while regime artillery has flattened the FSA’s headquarters in Aleppo, the ISIS camp next door was left untouched until the jihadi group left; the same is true in the fiercely contested eastern city of Raqqa. ISIS, for its part, has done very little to liberate regime-held areas, but has seized control of both Raqqa and the border town of Azaz from FSA forces.

Maybe it is just a conspiracy theory. Aaron Zelin, a Syria analyst who closely follows the dynamic among rebel groups, dismisses the idea as "partly wish-fulfillment and partly delusion." But there’s no mistaking the hydraulic effect of ISIS’s brand of uncompromising Islam. I spoke to a group of wounded fighters recovering in a clinic in the Turkish town of Reyhanli, a few miles from the border with Syria. One of them, who called himself Abu Abbas, had gone to al-Baath University in Homs with my interpreter, Rifaie Tammas. He had been pursuing a master’s degree in English literature. He shocked Rifaie by defending ISIS and claiming that the group is fighting the moderates because they are American stooges. The only answer for Syria, said Abu Abbas, is the rule of sharia.

The growing Islamization of the rebellion has something to do with ISIS, though a good deal more to do with the rebels’ growing sense of embitterment at their abandonment by the West and by exile groups who squabble among themselves in the comfort of Turkish or Egyptian hotels. The Islamists — not just ISIS — say, "We have no one to turn to but God," and young men like Abu Abbas have little reason to think otherwise.

The moderate rebels have become increasingly chimerical. On Sept. 24, 13 fighting groups — including the al Qaeda affiliate Jabhat al-Nusra, Salafi brigades, and some more mainline elements — issued a joint declaration in which they pledged to operate within an "Islamic framework" based on "the rule of sharia and making it the sole source of legislation." At the same time, the groups cut all ties with the Syrian National Council (SNC), the exile group that has received Western support. The pledge looked less like a gesture of solidarity than of despair.

It could have been otherwise. U.S. President Barack Obama could have bolstered moderate forces if he had supplied the rebels with weapons more than a year ago, as he was urged to do by Secretary of State Hillary Clinton and others. By refusing to do so, he discredited the SNC, alienated fighting units, and created a vacuum that ISIS has increasingly filled. Now, thanks in no small part to that failure, Obama has far stronger grounds to withhold U.S. military assistance than he had before. The president is not going to put much stock in a rebellion that puts so little stock in Western values.

What’s more, the fear that advanced weapons might fall into the hands of extremists, arguably overblown 18 months ago, is now impossible to discount. The fighters and activists I spoke to insist that the only way they can take on ISIS, as well as the regime, is with a steady supply of weapons and ammunition. They’re right, but they won’t win that argument in Washington. And the consequences of a hypothetical military victory look more and more dangerous. Imad Dahro, a former general in the national police who defected last year, assured me, as many people did, that the regime would collapse in the face of a sustained American missile strike. "Then what?" I asked. Wouldn’t the myriad rebel groups in the north then turn on each other? He reflected for a moment, and said, "Maybe."

The rise of ISIS, in short, has made the situation much worse for the rebels, much worse for the West, and much better for the regime. I heard any number of Syrians calling for nonradical brigades, with a core of Free Syrian Army groups, to join forces against ISIS. Only then, the argument runs, can they make a concerted effort to wage the war against the real enemy — the regime. What is certainly true is that the rebels will not get major help from the West unless and until they reverse the process of Islamization, though select brigades will continue to receive arms and ammunition from Saudi Arabia, Qatar, and others.

But radicalization is likely to increase, not diminish. Foreign extremists will keep streaming into Syria (a recent Der Spiegel article estimated the jihadi population of Atmeh, a Syrian town just across the border from Reyhanli, at 1,000). Assad will continue to exploit the focus on chemical weapons to commit atrocities against Syrian civilians. The rebels will keep absorbing and inflicting losses. And the endless torrent of refugees will further destabilize Lebanon and Jordan.

Obama has never tried to make the argument that America’s national interests lie in preventing such a debacle, through military as well as diplomatic means. And now, perhaps, it’s too late.