What's at stake for the players in Syria
Whom it backs: Moderate elements among the rebel forces.
Whom it opposes: The government of President Bashar Assad, as well as the Islamic State in Iraq and the Levant.
How it is fighting: The U.S. leads a coalition conducting airstrikes in Iraq and Syria against ISIL and other extremists. It has been carrying out a covert program to train and equip rebels.
Whom it backs: Assad, the leader of Syria, which has been Russia's only persistent ally in the Middle East for decades.
Whom it opposes: ISIL, which many Russians have joined.
How it is fighting: Russia supplies arms to Syria, but Russian pilots carried out their first airstrikes in Syria on Wednesday. Russia has deployed military equipment and soldiers to a Syrian airfield and its drones conduct reconnaissance flights.
Whom it backs: The U.S.-backed coalition.
Whom it opposes: The Assad government and Kurdish insurgent groups active in Turkey; nominally, also ISIL.
How it is fighting: Turkey began airstrikes and military incursions in July. It allows the U.S. coalition to use its bases.
Whom it backs: Assad and the Syrian government.
Whom it opposes: Sunni insurgents and ISIL.
How it is fighting: Iran is Syria's staunchest ally, and has been providing military support, weapons, supplies and financial aid since the start of the civil war in 2011.
Whom it backs: Rebel groups fighting Syria's government.
Whom it opposes: Assad and the Syrian government.
How it is fighting: Saudi Arabia will not accept a Russian effort to keep Assad in power. It funds and arms rebels and began conducting airstrikes against ISIL a year ago.
New York Times