The Syrian rebels have a serious problem with their best fighters. These are a few Islamic radical groups who contain the most fanatic and religiously intolerant rebels. Most of these fanatical fighters belong to one group, Jabhat al Nusra. This is an al Qaeda affiliate created in Syria six months ago. It has since been declared an international terrorist group by the United States and the European Union. The more moderate rebel groups and the Arab countries who support the rebels oppose this designation, but Nusra does spout the same radicalism al Qaeda favors and makes no secret of its opposition to democracy and fondness for a religious dictatorship in Syria once the current secular one is destroyed.
Nusra comprises 5-10 percent of the armed men fighting for the rebels. But because of their fanaticism they are more likely to carry out extremely daring and dangerous attacks (like suicide bombings). Nusra does most of its fighting the conventional way, just at a higher level of intensity and disregard for their own safety. To make that happen Nusra recruits Islamic fanatics from foreign countries (who comprise about half their strength). Most of the recruits are Arabs. About half of them come from nations that recently overthrew their governments via Arab Spring uprisings (Libya, Tunisia, Egypt). The rest come mostly from other Arab states plus Pakistan, Chechnya, Turkey, and Western nations (often ethnic Arabs whose parents moved there).