One reason Islamic terrorists have been so successful at setting up operations in Syria was because a lot of those terrorists, or at least their leaders, had been in Syria before (as terrorists) and had come to know the country, and its terrorist resources, well. This was no accident.
Since the 1980s, the Assad family has cultivated a reputation as benevolent dictators. The Assads paid a lot of attention to their media and diplomatic image. Unlike Kaddafi in Libya and Saddam in Iraq, the Assads were approachable and "reasonable." This despite the fact that the Assads long provided sanctuary and support for some of the worst terror groups on the planet. But they kept these connections quiet and got quite good at denying the presence of terrorist groups based in Syria. It was an open secret about all the terrorists operating out of Syria, but the Assads managed to avoid any sustained media criticism over this. Because of this the Assads had the support of most Arab governments, if only because the Assads pressured its guests not to attack other Arab states. The Assads were someone you could do business with, especially if it was dirty business you wanted to keep quiet. Another angle the Assad’s exploited was the fact most of the Syria based terrorists were focused on Israel. Hating Israel was popular in nearly all Arab countries.
After 2003, many of the terrorists hosted by the Assads were Sunni groups trying to overthrow the elected Shia government of Iraq. This was a very popular effort with most Arabs (who are Sunni). Iraq is one of those few Arab nations with a Shia majority. Sunni Arabs always felt better if Iraq was run by the Sunni Arab minority. Most Iraqis disagreed and the American invasion, despite protests from Sunni Arab states, brought elections that created the first Shia run Iraqi government in centuries. Their support of terrorism in Iraq cost Syria some good will in the Arab world, but the pro-Assad terror groups paid well for their sanctuary.
The great irony of the Syrian support for Sunni Arab terrorists in Iraq was that the Assads are Shia (or at least a semi-Shia sect called Alawites) and normally very effective (and violent) at suppressing any terrorists in Syria who attack the Assad government.
Moreover, the Assads are allies of Iran, the leader and "protector" of Shia everywhere. Iran is also seen as the main threat to the Sunni Arab world. Iran is a religious dictatorship with various factions, some of them willing to support Sunni Arab terrorists whose main goal was to kill lots of Iraqi Shias, in order to start a civil war that would put the Sunni Arab minority back in power. This was always a fantasy, and after years of terrorism and lots of blood, the Iraqi Sunni Arab tribes turned on the Sunni terrorists in 2007, and that was the end of that. The Iranian radicals tolerated all this slaughter in an attempt to get pro-Iran radical Iraqi Shia in power. That didn't work either, as most Iraqis had had enough of terrorism and violence and didn't want the kind of religious dictatorship that exists in Iran.
By 2007, many Sunni Arab terrorist leaders from Iraq and other Arab states had learned a lot about who could do what in Syria, and when the Arab Spring triggered a civil war in Syria two years ago, many terrorists found themselves operating in familiar territory. This time the Assad security forces were not able to promptly crush the Sunni terrorists attacking the Assads and it’s become all-out war between the Assads and their Sunni majority population. One of the most lethal rebel groups are the Sunni Islamic terror groups who seem to know their way around Syria quite well.