Having seen its star wane in Iraq, al Qaeda has staged a comeback in neighbouring Syria, posing a dilemma for the opposition fighting to remove President Bashar al-Assad and making the West balk at military backing for the revolt.
The rise of al Qaeda's affiliate in Syria, al-Nusra Front, which the United States designated a terrorist organisation last week, could usher in a long and deadly confrontation with the West, and perhaps Israel.
Inside Syria, the group is exploiting a widening sectarian rift to recruit Sunnis who saw themselves as disenfranchised by Assad's Alawite minority, an offshoot of Shi'ite Islam that dominates Syria's power and security structures.
Al-Nusra appears to have gained popularity in a country that has turned more religious as the uprising, mainly among Sunni Muslims, has been met with increasing force by authorities.
It has claimed responsibility for spectacular and deadly bombings in Damascus and Aleppo, and its fighters have joined other rebel brigades in attacks on Assad's forces.
According to Site Intelligence group, Nusra claimed responsibility in one day alone last month for 45 attacks in Damascus, Deraa, Hama and Homs provinces that reportedly killed dozens, including 60 in a single suicide bombing.