The designation of Jabhat Al-Nusra as a terrorist organization by the US doesn't convince me. The Obama administration had known for a long time what kind of guys these are and they had done nothing. Recent news reports in which Al-Nusra openly showed off its Al Qaeda ancestry left Obama no choice. Another reason may have been some agreement with Russia about solving Syria. But it is only an empty show. There will be no pressure on the Gulf States and Turkey to isolate Al-Nusra and it is not unlikely that in a later stage Al-Nusra will be rehabilitated - as happened with the Mujahadeen-e-Khalq. The US was also happy to support former members of Al Qaeda related LIFG in the Libyan uprising.
It looks like militarily there is no real alternative for Al-Nusra. The FSA has never become the centralized command that it was supposed to be and with the recent military successes its associated militias are increasingly inundated with opportunists and outright criminals. The consequence is increasing misbehavior and corruption. As a consequence its popularity with the population is sinking.
So a military victory for the rebels looks unlikely without the contribution of Al-Nusra. Thanks to that and to the presence of a multitude of other Jihadi rebel organizations many rebel fighters in Syria have condemned their designation by the US as a terrorist organization.
It seems unlikely that the rebel supporters in Washington, Ankara, Qatar and Saudi Arabia are prepared to give up on the rebels because of Al-Nusra. If they might isolate Al-Nusra anyway - because popular indignation in the West became too high - they would likely appoint another of the many Jihadi rebel organizations in Syria to take over its functions.
In the mean time Al-Nusra has recognized that it has an image problem and is trying to improve that. Until recently its fighters lived isolated from the population in training camps, tried to limit contact with the outside world and refused interviews. Now they have given their first interview and in that interview the downplayed their connections with Al Qaeda - that they had previously flaunted. They are also getting more involved with their surroundings - a.o. organizing food distribution.
Yet Al-Nusra bodes ill for the future of Syria. Unlike what many Syrian rebels like to believe Jihadi's don't believe in democracy and tend to be serious about establishing an Islamist state. And they won't have any qualms about blowing up moderate Sunni politicians when they think it might further their case.
In the mean time the Jihadi's keep expanding and their next target may be Lebanon.
This article in Spiegel claims that Al-Nusra doesn't exist as a central organization. Al Nusra first became public with some bomb attacks in Damascus that some claimed were done by Assad supporters to turn the population against the rebels. According to the article some local rebel cells have taken up this name to radiate strength. It seems to more probable that the journalist has fallen for some rebel propaganda that tries to counter the damage of the terrorist designation. The rest of the article looks at the situation in Syria with a very rosy look of the rebels.
Quilliam Foundation has published a study about Al-Nusra: Jabhat al-Nusra: A Strategic Briefing. Here is a summary. Quilliam is a counter-extremism think tank, founded by former ideologues of UK-based extremist Islamist organizations.
Some reporting about Al Nusra in After Assad, is strict Islamic rule ahead for Syria?: One former member, Abu Osama, said he left the group after it tried to get him to sign an oath pledging to fight with the group anywhere in the world. [..] The group, which forbids tobacco use, has also been known to pull cigarettes out of the mouths of smokers going through their checkpoints.
Here is a BBC interview with an Al Nusra member and some related reporting.
According to Syria Deeply: Analysts say Jabhat al Nusra’s success in battle comes from having experienced fighters. Activists tell Syria Deeply that up to 20% of the group’s ranks are foreign fighters, coming from Libya, Iraq, and other Arab countries.