CAIRO — In Syria’s largest city, Aleppo, rebels aligned with Al Qaeda control the power plant, run the bakeries and head a court that applies Islamic law. Elsewhere, they have seized government oil fields, put employees back to work and now profit from the crude they produce.

Across Syria, rebel-held areas are dotted with Islamic courts staffed by lawyers and clerics, and by fighting brigades led by extremists. Even the Supreme Military Council, the umbrella rebel organization whose formation the West had hoped would sideline radical groups, is stocked with commanders who want to infuse Islamic law into a future Syrian government.

Nowhere in rebel-controlled Syria is there a secular fighting force to speak of.

This is the landscape President Obama confronts as he considers how to respond to growing evidence that Syrian officials have used chemical weapons, crossing a “red line” he had set. More than two years of violence have radicalized the armed opposition fighting the government of President Bashar al-Assad, leaving few groups that both share the political vision of the United States and have the military might to push it forward.

Among the most extreme groups is the notorious Al Nusra Front, the Qaeda-aligned force declared a terrorist organization by the United States, but other groups share aspects of its Islamist ideology in varying degrees.