What's at stake in Syria

United States

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.

Russia

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.

Turkey

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.

Iran

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.

Saudi Arabia

Whom it backs: Rebel groups fighting Syria's government.

Whom it opposes: Assad and the Syrian government.