Here is a breakdown of the forces involved in the war, which has killed more than 350,000 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 by Iranian, Iraqi and Afghan fighters.
Regime backer Russia launched an air campaign in support of President Bashar al-Assad in September 2015, helping Damascus recapture key areas including second city Aleppo and the rebel enclave of Eastern Ghouta outside Damascus.
Iran has also given Assad major financial and military support.
The Syrian government now holds major cities Damascus, Aleppo, Homs and Hama, and around 70 percent of the population lives in regime-held areas.
Syria's opposition comprises multiple factions including moderate rebels and Islamist groups, but it has shrunk from estimates of up to 100,000 fighters as the government has retaken territory.
Early in the uprising, rebels coalesced under the banner of the Free Syrian Army (FSA), but the opposition has since splintered.
Since mid-February, an array of different rebel groups have lost their onetime bastion of Eastern Ghouta on the edge of the capital.
The jihadist Hayat Tahrir al-Sham is a coalition essentially composed of Al-Qaeda's former affiliate.
Two major rival jihadist forces operate in Syria: the ISIS and Hayat Tahrir al-Sham.
ISIS emerged from wars in Syria and neighbouring 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, ISIS has suffered major territorial losses under pressure from a US-backed alliance of Kurdish and Arab fighters, as well as Russian-backed regime offensives.
Meanwhile, the Hayat Tahrir al-Sham jihadist coalition controls most of the northwestern province of Idlib.
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 have been a key partner of the US-led coalition fighting ISIS and forms the backbone of the Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab fighters.
Since January, the Kurdish militia have also been fighting pro-Ankara forces in northwestern Syria, pulling SDF forces away from the battle against the jihadists in the east of the country.
Turkey-led fighters last month overran the former Kurdish enclave of Afrin on the Turkish border, and Turkey has threatened to take the battle further east.
But the YPG still controls 28 percent of Syrian territory, including large parts of the northern border with Turkey.
Around 15 percent of Syria's population lives under Kurdish rule.
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.
Today, Riyadh and Doha's roles have been reduced, with Ankara taking the lead as international supporter of the opposition in negotiations on ending the conflict.
Along with Iran, Turkey and Russia last year worked closely to implement four so-called "de-escalation zones" around the country.
On the ground, Turkey has battled both IS and Kurdish forces it accuses of being "terrorists", most recently in Afrin.
It is also present in Idlib.
A US-led coalition has targeted ISIS 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 was a key backer of the SDF advance on Raqa, and has supported the militia in an operation against IS its last desert holdouts in the neighbouring province of Deir Ezzor.