CAIRO -- In Syria's largest city, Aleppo, rebels aligned with al-Qaida 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 Barack 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 has radicalized the armed opposition fighting the government of President Bashar Assad, leaving few groups with both a political vision the United States shares and the military might to push it forward.
Among the most extreme is the notorious Al Nusra Front, the al-Qaida-aligned force declared a terrorist organization by the United States, but other groups also share aspects of its Islamist ideology in varying degrees.
"Some of the more extremist opposition is very scary from an American perspective and that presents us with all sorts of problems," said Ari Ratner, a fellow at the Truman National Security Project and former Middle East adviser for the State Department. "We have no illusions about the prospect of engaging with the Assad regime -- it must still go -- but we are also very reticent to support the more hard-line rebels."
Syrian officials recognize that the United States is worried that it has few natural allies in the armed opposition and have tried to exploit that with a public campaign to persuade, or frighten, Washington into staying out of the fight. At every turn, they promote the notion that the alternative to Mr. Assad is an extremist Islamic state.
The Islamist character of the opposition reflects the main constituency of the rebellion, which has been led since its start by Syria's Sunni Muslim majority, mostly in conservative, marginalized areas. The descent into brutal civil war has hardened sectarian differences and the failure of more mainstream rebel groups to secure regular arms supplies has allowed Islamists to fill the void and win supporters.
The religious agenda of the combatants sets them apart from many civilian activists, protesters and aid workers who hoped the uprising would create a civil, democratic Syria.
When the armed rebellion began, defectors from the government's staunchly secular army formed the vanguard. The rebel movement has since grown to include fighters with a wide range of views, including al-Qaida-aligned jihadis seeking to establish an Islamic emirate, political Islamists inspired by the Muslim Brotherhood and others who want an Islamic-influenced legal code like that found in many Arab states.
"My sense is that there are no seculars," said Elizabeth O'Bagy, of the Institute for the Study of War, who has made numerous trips to Syria in recent months to interview rebel commanders.
Of most concern to the United States is the Nusra Front, whose leader recently confirmed that the group cooperated with al-Qaida in Iraq and pledged fealty to al-Qaida's top leader, Ayman al-Zawahri, Osama bin Laden's longtime deputy. Nusra has claimed responsibility for a number of suicide bombings and is the group of choice for the foreign jihadis pouring into Syria.
Another prominent group, Ahrar al-Sham, shares much of Nusra's extremist ideology but is mostly made up of Syrians.
The two groups are most active in the north and east and are widely respected among other rebels for their fighting abilities and their ample arsenal, much of it given by sympathetic donors in the Persian Gulf.