The Conflict:

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.

Internal Players:

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

Outside Stakeholders:

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