Recent claims that chemical weapons have been used in Syria's civil war have increased the pressure on U.S. President Barack Obama to intervene in the conflict. In addition to considering a no-fly zone, he is said to be weighing whether to provide rebel fighters with weapons.
Who are the fighters in Syria's insurgency?
The majority are from the Free Syrian Army, a force of some 70,000 defectors from the Syrian military and volunteers who have taken up arms against the regime. They're part of a loose coalition of 30 armed groups recently brought under a joint military command led by General Salam Idris, a senior Syrian military officer who defected.
Not in the coalition are Islamic militant groups who are also trying to oust President Bashar al-Assad. Many of these fight under the banner of the Syrian Islamic Front, an umbrella group for several hard-line brigades.
Taken together, says Michael O'Hanlon of the Brookings Institution, Syria's opposition fighters are a fractious force with a variety of axes to grind against a regime dominated by Assad's Alawite sect, an offshoot of Shi'ite Islam.
"A number of them are Al-Qaeda linked, especially the Al-Nusra group, and on top of that, we have a number of groups that -- perhaps understandably, but still dangerously -- are extremely embittered by many years or decades of oppression," O'Hanlon says.
"They feel like they -- most of them being Sunnis -- have been systematically deprived of their rights by the Alawite-[dominated] regime, and they intend to settle scores. Then there are real Syrian patriots, and I like to hope that's the largest fraction."
How big of a factor are the Islamic militant groups in the U.S. debate over whether to give weapons to the rebels?
Judging from statements by Obama, Joint Chiefs of Staff Chairman General Martin Dempsey, and other U.S. officials, their presence in the rebel force is fueling the White House's indecision. They are the wild card in an already high-stakes poker game.
Recently, the leader of the strongest jihadist group, the Al-Nusra Front, pledged allegiance to Al-Qaeda leader Ayman al-Zawahri, who has a $25 million U.S. bounty on his head. Predictably, the move raised alarm in Washington, but surprisingly, it also angered other Islamist fighters in Syria.
The Syrian Islamic Liberation Front, which represents several militant groups, said Al-Nusra had pledged loyalty "to someone who does not understand our reality and does not serve our people or nation."
That statement reinforced U.S. perceptions that there is discord even among like-minded rebel groups. And that, O'Hanlon says, is another factor complicating Washington's next move: uncertainty over what will happen if Assad falls.
If and when that happens, the single thread tying all the factions together will be gone. The resulting power vacuum could easily spark another conflict.
"These are groups that have come together in the fairly recent past with a specific military goal in mind. The war has been very tough. They're angry, they're embittered, they're not even happy with us anymore because they think we've betrayed them. They certainly feel Assad's betrayed them," O'Hanlon explains.
"They're probably nervous about each other. The literature on civil war suggests that groups that fight together against a common enemy may ultimately turn on each other if things don't work a certain way."
There's a lot of talk of picking the "right" fighters, but how does the United States do that?
It's all about relationship building, according to O'Hanlon. Foreign governments looking to tilt the balance in the war need to get "into the trenches" with the rebel fighters and get to know them, he says.
Only then can Washington and its allies use incentives like military aid to influence how the various factions work toward their shared goal of regime change.
"I think we have to view this as an ongoing project. We don't just say, 'Here are the 17 groups we'll help and there are the six we won't, and let's assume we got it right once and for all and we can otherwise just keep our hands off.' We've got to stay engaged and try to get more groups to try and behave in an acceptable way," O'Hanlon says.
Couldn't U.S. weapons eventually end up in the wrong hands and be used against U.S. interests?
This is one of Obama's biggest concerns, with good reason. Some weapons sent by the United States and its allies to Afghan mujahedin in the 1980s later ended up in the hands
of Islamic extremists.
Controlling the chain of custody once arms are transferred might be impossible, but O'Hanlon believes the United States can take steps to limit the potential blowback.
"We probably have to limit the degree to which we would give any advanced weaponry, like advanced surface-to-air missiles," he says. "If you do wind up providing that sort of thing, you probably have to look for ways to modify them so they have only very short lifetimes with their batteries or maybe even self-destruct after a certain period of time because we can't have these things floating around the world, taking down commercial airliners."
If the United States does decide to send military aid, it won't be the first country to do so: Saudi Arabia, Turkey, and Qatar have been quietly supplying the rebels for months.