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At Damascus airport, he said, they were easy to pick out: “Bearded. One-way ticket. Very little luggage.”

The Combating Terrorism Center issued a study in 2007 that looked at about 700 foreign fighters, based on records seized by the U.S. in a village near the Syrian border.

All those listed had come to Iraq via Syria within one year, an indication of a sophisticated operation that is being reversed as fighters move into Syria from Iraq.

“For Syria, supporting jihadi groups is at best a double-edged sword,” the West Point report says.

The Congressional Research Service reported in June: “Many analysts fear that Al Qaeda in Iraq (AQ-I) also has been active in the Syrian civil war, supporting and training Sunni extremist groups in Syria.”

Asked to assess Al Nusra in its second year, a U.S. official said: “The Nusra Front is clearly playing an important role in the opposition because the group’s roughly 6,000 fighters are disciplined, well-armed and capable. They’ve also partnered effectively with other rebel brigades. However, Nusra is still just one part of a complex, multipronged opposition that doesn’t have one dominant force.”

The Nusra Front’s rise is one reason liberals and conservatives question the Obama’s administration’s decision to provide small arms to selected rebels, bringing the risk that al Qaeda might end up with them.

“Basically, it’s not a matter of size but of determination and resources,” said Michael Rubin, a former Pentagon policymaker and an analyst at the American Enterprise Institute. “Al Nusra isn’t a Syrian organization. It is international.

“On jihadi websites, you can read the obituaries of Saudi, Turkish and even Swedish and German fighters. If you want two words to reinforce why arming the Syrian opposition is such a bad idea, they are ‘Nusra Front.’”