Here is a breakdown of the forces involved in the war, which has killed over 320,000 people:
Regime and allies
Syria's 300,000-strong pre-war army has been halved by deaths, defections and draft-dodging.
It is bolstered by up to 200,000 irregulars and as many as 8,000 men from Lebanon's Shiite Hezbollah movement as well as Iranian, Iraqi and Afghan fighters.
Regime backer Russia began an air campaign in support of President Bashar al-Assad in September 2015, helping Damascus recapture key areas including second city Aleppo.
Iran has also given Assad major financial and military support.
The government now holds around half of the country.
Rebels
Syria's opposition comprises multiple factions including moderate rebels and Islamist groups.
Estimates of total opposition forces range from tens of thousands up to around 100,000.
Early in the uprising, rebels coalesced under the banner of the Free Syrian Army (FSA), but the opposition has since splintered.
One of the most powerful factions is Ahrar al-Sham, which espouses a hardline Islamist ideology and is strong in Idlib province.
Another leading Islamist rebel group, Jaish al-Islam (Army of Islam), is active around Damascus.
Rebels who once controlled large areas now only hold around 12 percent of the country, according to Syrian geography expert Fabrice Balanche.
That includes areas where they are allied with the jihadist Fateh al-Sham Front.
Around 12.5 percent of Syria's remaining population lives in rebel-held territory.
Jihadists
Two major rival jihadist forces operate in Syria: the Islamic State group and former Al-Qaeda affiliate Fateh al-Sham Front.
IS emerged from wars in Syria and Iraq to seize swathes of both countries in mid-2014.
It declared an Islamic "caliphate", committed widespread atrocities and carried out or inspired deadly attacks around the world.
Since then, IS has suffered major losses under pressure from an air war by a US-led coalition.
It holds around 20 percent of Syrian territory, though that is rapidly shrinking as regime forces and a US-backed alliance of Kurdish and Arab fighters advance against it.
Fateh al-Sham Front split in July 2016 from Al-Qaeda and has been close to Ahrar al-Sham since 2015, though infighting has also erupted between the two factions.
The Kurds
Syria's Kurds have largely avoided the conflict between the government and armed opposition, carving out a semi-autonomous region in northern and northeastern Syria.
Their People's Protection Units (YPG) militia became a key partner of the US-led coalition fighting IS and forms the backbone of the US-backed Kurdish-Arab Syrian Democratic Forces (SDF).
The YPG controls about 20 percent of Syrian territory but some three-quarters of the northern border with Turkey.
Over two million people live in Kurdish-held Syrian territory.
The SDF began an operation to capture IS stronghold Raqa in November 2016, entering the city in June with US-led coalition support.
Turkey began an offensive into Syria in August 2016 against both IS and the YPG, which Ankara regards as the Syrian branch of the Kurdistan Workers' Party (PKK) which has waged a 33-year insurrection inside Turkey.
Turkey, Saudi Arabia, Qatar
Sunni-majority Saudi Arabia, Qatar and Turkey have provided military and financial support to rebels fighting Assad, who belongs to the minority Alawite sect linked to Shiite Islam.
Long accused of turning a blind eye to jihadist activity along its southern border, Turkey has joined the US-led coalition fighting IS and deployed troops to battle the group in northern Syria.
Although they support opposing sides, Ankara and Moscow have worked closely in recent months to reach a political solution to the conflict.
International coalition
A US-led coalition has targeted IS and other jihadists in Syria with air strikes since 2014.
The coalition includes Britain, France, Saudi Arabia and Turkey along with Australia, Bahrain, Canada, Jordan, the Netherlands and the United Arab Emirates.
It is a key backer of the SDF advance on Raqa, but has been drawn into clashes with regime forces also advancing on the city since early June.
On June 18, a US warplane downed a Syrian fighter jet that had allegedly targeted SDF fighters south of Raqa.
That followed several incidents in which US forces targeted pro-regime fighters who approached a base near the Iraqi border where the US trains anti-IS forces.
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