Forget the geopolitical stuff, all you need to know is the Command & Conquer style faction stats.
ISIS-Has tons of armored vehicles and US military equipment stolen from Iraqi forces...and no logistical base with which to supply/use them. As a result, their typical vehicle is a 1990's Toyota with a heavy machine gun in the bed. What they would really, really like for Christmas is some 23mm cannons to chase those Hinds off with. They're very much a Mao Zedong style propaganda centric fanatical human wave army.
Al-Nusra-Al Qaeda in Syria, the group that absorbed the "Syrian Rebels" and "Free Syrian Army" "moderate rebels" etc when ISIS showed up. Less fanatical and scrappy than ISIS, they're currently the US's BFF in the region. In addition to their vintage AK-74's, they're rocking US equipment, TOW missiles and some older MANPADs, enough to knock out armor and even flying vehicles if they're not careful.
SAA-This the official Syrian army, and Russia's BFF in the region. They have uniforms, training (kind of), and all manner of 1970's and 1980's era armor and equipment. It's like a real army, but more heavy-handed than your average Western army because they think fighting Jihadis makes the ends justify the means. You'll typically see them in all manner of vehicles getting blown up by TOW missiles and getting bailed out by the Russian air force.
Russia-Russia brought all kinds of toys with them to Syria, and thanks to Putin's military reforms they're not all blowing up the first time someone lights up a cig next to them. They're stuffed to the brim with countermeasures against those MANPADs so they can just fly around and play Call of Duty. In addition to an endless supply of 50 year old dumb bombs, Russia brought their SAMs and set them up over Syria. The Russian military is hit and miss, but their SAMs are the best in the world, and everyone is quickly getting tired of having to ask Russia's permission to fly over the region.
