Syria has been stuck in a war involving several domestic and foreign forces for nearly five years with almost no common ground to be found among the main organizations fighting on the ground.
Al-Assad government: President Bashar al-Assad's forces still control most of the large cities such as Damascus, Homs and parts of Aleppo as well as the western coastal strip. Syria's army has suffered heavy losses, but al-Assad has refused to step down.
Islamic State: The extremist group controls vast areas in northern and eastern Syria and has set up local administrations. The militia has suffered territorial losses in recent months, especially in the Kurdish regions, but remains the most powerful non-governmental force.
Rebels: Rebel strongholds are in the north-west and south. They include some moderate groups that are supported by the West but are increasingly dominated by Islamic hardliners such as Jaish al-Islam and Ahrar ash-Sham. Some of them also cooperate with al-Qaeda-linked al-Nusra Front. Government airstrikes are putting them on the back foot.
Opposition: The opposition is fragmented. The most important alliance in the opposition is the Syrian National Coalition. In Damascus there are also opposition parties, which are tolerated by al-Assad's regime. The Higher Negotiations Committee is an opposition grouping formed in Saudi Arabia for the purpose of negotiations, but many prominent representatives of the opposition are not included in the panel.
The Kurds: They control most of the border with Turkey. The Kurdish Democratic Union Party (PYD) and its fighters are the most effective ground force beating back Islamic State and are allied with the United States. Many rebels, who dislike the Kurds' secular-leftist ideology, accuse them of working with al-Assad, a charge they deny. The PYD has links to the armed Kurdistan Workers' Party (PKK), which is banned in Turkey. The party set up autonomous zones but denies it seeks to secede from Syria.
United States: Washington is leading an international coalition against Islamic State, which includes daily airstrikes. US allies participating in the airstrikes include France, Britain, Australia, Bahrain, Canada, Jordan, Saudi Arabia, Turkey and the United Arab Emirates. Germany is supplying reconnaissance planes. Washington also supports some moderate opponents of al-Assad's regime.
Russia: A main backer of al-Assad, Russia has been flying bombing campaigns to support the government since September. Moscow says its raids are aimed at Islamic State and other extremist groups, but rebels say they have borne the brunt.
Iran: Tehran is the most loyal supporter of the Syrian government. Iran has recruited its own citizens and others to support al-Assad's soldiers. Hezbollah, a Lebanese Shiite movement financed by Iran, is also fighting along with al-Assad's troops.
Saudi Arabia and Turkey: The two regional powers are important supporters of the rebels. They both demand al-Assad's resignation. Saudi Arabia hopes to diminish the influence of Iran, its arch-enemy in the Middle East. Turkey is seeking to contain Kurdish gains in Syria.
Israel: Israeli airstrikes - which the government rarely confirms - appear to have primarily targeted Hezbollah and arms transfers to the group's bases in Lebanon. Israel has given some humanitarian assistance to rebel fighters near its border.