The accusation that Sen. John McCain unwittingly met with terrorists while in Syria to talk with Western-backed rebel forces remains unproven -- but it underscores the challenge the United States faces in figuring out exactly whom to support in efforts to oust Syrian dictator Bashar al-Assad.
More than two years after the civil war started, we don't know who the opposition really is.

The Free Syrian Army, visited by McCain last month and started largely by defectors from al-Assad’s military forces, has emerged as the group most likely to receive Western military aid, considering its stated, non-political, non-religious goal of toppling the current regime.

However, groups like the Muslim Brotherhood and varying factions within the FSA and in the smaller, lesser-known rebel organizations present a complicated and confusing situation for the U.S. and its allies.

The overriding question now is deciding whether and how to supply them with weapons, and whether to trust that a post-Assad Syria will be better.

Critics of intervention, like Sen. Rand Paul, R-Ky., cite post-Qaddafi Libya as an example of how the leadership vacuum can stoke chaos. Syria's population is even more diverse and difficult to read.

“It’s a mosaic of religious and ethnic sects,” said Jim Phillips, a Middle Eastern affairs expert with the Heritage Foundation. “As the so-called Arab Spring continues, the groups that rise up and try to overthrow governments in the region are also divided by ideological differences.”

The FSA, for example, is made up mostly of Sunni Arabs but is essentially a volunteer umbrella organization that includes members of the Islamic group Alawites and platoons of Palestinians and Kurds -- a Sunni Muslim group.

Meanwhile, smaller rebel groups have also entered the two-year civil war, streaming across the Syrian border from such neighboring countries as Turkey, Iran, Lebanon and Libya, home to Sunni militants opposed to the Hezbollah forces that are helping al-Assad retain power in a war in which at least 70,000 civilians and others have already died.

Phillips says the foreign groups and others are driven by a “hardcore militant ideology” and the possibility of replacing al-Assad’s non-secular government with stricter Islamic law known as Sharia.
“They are like moths to flame,” he said.

If there is indeed consensus among U.S. and its allies about who not to arm in the two-year civil war, it would be the Al-Nusra Front.

Though experienced in guerilla warfare and effective against al-Assad forces, Al-Nusra is a known terrorist group that the United Nations Security Council last month declared a front for Al Qaeda in Iraq.

A newly released report found many of the foreign fighters killed in Syria are with Al-Nusra and from Libya.

The 10-month review by FlashPoint Global Partners, a leading terrorism research group, also found the emergence of foreign fighters -- whether from Syria, Iraq or the tribal areas of Pakistan -- is considered a sign that the jihadi cause or ideology is taking hold in the conflict.

Michael Rubin, a former Pentagon official and Middle East expert with the American Enterprise Institute, points out that trying to determine whether a person or group is connected to a larger, radical organization is difficult for intelligence officials, even on their own soil.

“We couldn’t adequately vet two Chechens living in Boston,” he said, referring to the Boston Marathon bombers.

Rubin also argues that it's nearly impossible to identify “card-carrying members” of any group in a multi-faceted war roughly 5,500 miles away.

“It’s really free agents,” he said. “Those coming in don’t have the idea of group membership like we do.”

Rubin argues the United States has done a good job in deciding who to trust in places like Afghanistan, taking as long three years to investigate and approve a defense contractor.

“But I don’t think anybody would argue we have that long in Syria,” said Rubin, adding that sources, including paid CIA informants, say one thing and do another.

As recently as Tuesday, the White House, on the issue of multiple reports of Assad using chemical weapons, made clear the administration’s cautious position.

The president has called the use of chemical weapons a "red line" that could trigger greater intervention, but has not acted yet on these reports.

“The president believes it's highly important to get this right,” said Press Secretary Jay Carney. “The American people expect the president and Congress to get this right.”

A Senate committee recently passed a bipartisan bill to give military aid to rebels, in hopes of spurring action, but made sure the vetting process was a key component.

“Sitting on our hands and not getting involved, it’s almost assured that Al Qaeda or at least extremists with similar views are going to control the country,” said Tennessee Republican Sen. Bob Corker, a cosponsor. “That’s what we are trying to prevent.”

Phillips argues the Obama administration’s stance has meanwhile allowed other countries to push their own agendas and that such inertia will open the door to further extremism and terrorism.

“One negative implication of the administration’s ‘lead from behind’ efforts … is that U.S. allies have independently stepped forward to advance their own interests by backing various rival groups,” he said.

 “If Washington continues its hands-off policy … then Syria is likely to devolve into an anarchic patchwork of warring fiefdoms that will provide fertile ground for Al Qaeda and other Islamist extremist organizations.”