As Syrian rebel commander Hamza al-Shamali describes the battle inside Syria, a few miles across the border from Reyhanli, Turkey, where we met, the immediate problem isn't defeating the Islamic State. It's coordinating the ragtag brigades of the Free Syrian Army into a coherent force that can fill the vacuum once the extremists are driven out.

"At some point, the Syrian street lost trust in the Free Syrian Army," he tells me. Shamali explains that many rebel commanders aren't disciplined, their fighters aren't well-trained and the loose umbrella organization of the FSA lacks command and control. The extremists of the Islamic State and Jabhat al-Nusra have filled the vacuum. Now, he says, "the question every Syrian has for the opposition is: Are you going to bring chaos or order?"

Shamali is the leader of a group called Harakat Hazm, or "Steadfastness Movement," which is the biggest U.S.-backed rebel force in Syria. He commands about 4,200 trained and vetted fighters. He's a lean man, tight as a coiled spring, with a thin beard and eyes hardened by three years of war that killed two of his brothers among nearly 200,000 Syrians who have perished.

The war is just over the Syrian border that bounds the southern edge of Reyhanli, about 525 miles southeast of Istanbul. The town has become a staging point for the rebels; people in the streets often speak Arabic with a Syrian accent, and many cars still have their Syrian license plates. This is where Syrian rebel groups maintain what passes for a military operations center.

In a safe house here, Shamali and his key deputies last weekend gave me the clearest account I've heard of the challenge ahead for the Obama administration as it tries to build a force that can "degrade and ultimately destroy" the Islamic State inside Syria. The problem is that the "moderate opposition" that the U.S. is backing is still largely a fantasy.

Shamali argues that rather than try to combine the motley brigades of the FSA, as some are urging, the opposition should create a new "Syrian national army" that can defeat the extremists and eventually topple President Bashar al-Assad. "We refuse to repeat failed experiments," he says, explaining why he rejected a merger proposed last week by former opposition leader Ahmad al-Jarba. The proposal to combine existing rebel brigades "is a cut and paste of previous FSA failures," he warns.

The interview with Shamali offered a rare glimpse inside the group the U.S. has supported under its nominally "covert" program to train and vet Syrian rebels. Formed last January, Harakat Hazm was the first group to receive U.S. anti-tank missiles; it also has the beginnings of an intelligence network and counterterrorism capability. The U.S. provides $150 a month for each fighter, and has recorded their biometric data.

Shamali says he's building a mobile guerrilla force in northern Syria, rather than attempting to hold local territory, as most of the opposition groups do. "You need a strike force - the tip of the spear - that can move very fast." Then he wants to train local people to "fill the void" as the extremists retreat. Shamali says he would fold his operation into a real national rebel army as soon as it's formed.

The FSA's biggest problem has been internecine feuding. Over the past two years, I've interviewed various people who tried to become leaders, such as: Abdul-Jabbar Akaidi, Salim Idriss and Jamal Maarouf. They all talked about unifying the opposition but none succeeded. An Arab intelligence source explains: "Until now, the FSA is a kind of mafia. Everyone wants to be head. People inside Syria are tired of this mafia. There is no structure. It's nothing." And this from one of the people who have struggled the past three years to organize the resistance.

The puzzle of creating the right structure for training and assisting the moderate opposition will fall largely to Gen. John Allen, a retired Marine who serves as President Obama's special envoy for Iraq and Syria. He'll be meeting in Jordan next week with members of the Syrian opposition.

In framing its Syria strategy, the Obama administration has to face up to a basic political problem, as well as the organizational issues. Most Syrian rebels are fighting because they hate Assad's regime. They have come to oppose the Islamic State, too, and many rebels appear ready to fight the extremists. But if U.S. airstrikes and other support are seen to be hitting Muslim fighters only, and strengthening the despised Assad, this strategy for creating a "moderate opposition" will likely fail.