The 2011 popular uprising against President Bashar al Assad quickly devolved into a sectarian conflict between the minority Alawite government and majority Sunnis, complicated by multiple outside actors with contradictory goals.
Free Syrian Army (FSA) - Loose, largely marginalized coalition of opposition military forces, including Assad defectors; mostly Sunnis, some Kurds and Alawites
Islamic State – Salafist jihadis holding large areas of Syria and Iraq. Led by Abu Bakr al-Baghdadi, self-declared caliph of self-styled caliphate
Jabhat al-Nusra - Al Qaida-affiliated Sunni militia fighting Assad regime, among others. Sent to Syria by then-AQ in Iraq head al-Baghdadi in late 2011
Army of Conquest – Newly-formed anti-Assad coalition; Nusra and other Islamists dominate
PKK – anti-IS Kurdish militias; tacit agreements with other battlefield forces
United States – gives open and covert support to some anti-Assad forces; leads anti-IS coalition, which includes UK, France, Saudi Arabia, Qatar and to a limited degree, Turkey
Iran – backs Assad with weapons, intelligence, funding and its Lebanon-based proxy Hezbollah militia, which has fought for Assad in Syria.
Russia – long-time Assad supporter, beefed up military presence in August 2015, began airstrikes against anti-Assad targets the next month