"Can anyone stop the radicalization of Syrian rebels?"

Foreign Policy

"... 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,"..
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.
..... 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....
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). ..."