Beirut - A new ceasefire brokered by Russia and the United States is due to begin at sundown on Monday in Syria.
It aims to halt fighting between President Bashar Assad's forces and the opposition, but does not apply to jihadists like the Islamic State group.
The regime and its allies have backed the truce but with only a few hours to go on Monday the opposition had not yet signed on.
Here is a breakdown of the forces involved:
The Syrian army's 300 000-strong pre-war force has been halved by deaths, defections and draft-dodging but is continuing to battle myriad rebel groups and jihadists.
The army is bolstered by 200 000 irregular fighters, notably from the National Defence Forces. It also battles alongside 5 000-8 000 men from Lebanon's powerful Shi’ite militia Hezbollah, as well as Iranian, Iraqi and Afghan fighters.
Russia, a key regime backer, began an aerial campaign in support of Assad's government in September last year and has helped Damascus recapture areas in several provinces.
Iran is another key ally, providing financial and military support.
Rebels and backers
The truce is to apply to a wide range of opposition forces, including moderate rebel fighters and Islamist factions.
The total number of rebel fighters is unclear, though in 2013 US Secretary of State John Kerry said there were 70 000 to 100 000 "oppositionists" fighting in Syria.
The Free Syrian Army (FSA) was a key opposition faction in the early days of the conflict but has since splintered into a range of groups, though the term is still often used to describe moderate rebels.
Ahrar al-Sham is Syria's most powerful non-jihadist rebel group, with a commanding presence in Idlib and Aleppo provinces.
It espouses a hardline Islamist ideology, and is a key partner of the jihadist Fateh al-Sham Front, participating in the Army of Conquest alliance with the group in ruling Idlib province.
Opposition factions deemed "moderate" are backed by the West, particularly the United States, France and Britain. Turkey, Saudi Arabia and Qatar also back the opposition, and have lent support to Islamist factions.
Another key opposition group is the Saudi-backed Jaish al-Islam (Army of Islam) linked to Mohammed Alloush, a leading rebel figure who briefly acted as the chief negotiator for the main opposition umbrella group, the High Negotiations Committee (HNC).
The Islamic State group and the Fateh al-Sham Front are excluded from the ceasefire agreement.
ISIS emerged from the chaos of the civil war to seize control of large parts of Syria and Iraq in mid-2014, declaring an Islamic "caliphate," committing widespread atrocities and carrying out or inspiring deadly attacks abroad.
Under pressure from a US-led air war launched two years ago and fighting on multiple fronts, ISIS has since suffered major losses but remains in control of significant territory in northern Syria, including its de facto capital Raqa.
The Fateh al-Sham Front is the former al-Qaeda affiliate in Syria and was previously known as Al-Nusra Front. It split in July from the global jihadist network founded by Osama bin Laden, in a move analysts said was aimed at easing pressure from both Moscow and Washington.
Questions remain about how the ceasefire will apply in several parts of the country where the Fateh al-Sham Front cooperates closely with rebel forces, including moderates and Ahrar al-Sham.
If the ceasefire holds for a week, Moscow and Washington are to begin unprecedented joint targeting of jihadist forces including ISIS and the Fateh al-Sham Front.
Syria's Kurds have largely stayed out of the conflict between the government and armed opposition, carving out a semi-autonomous region in north and northeastern Syria.
Their People's Protection Units (YPG) have become a key partner of the US-led coalition fighting ISIS as part of the Kurdish-Arab Syrian Democratic Forces.
Both the YPG and the SDF said on Monday that they would respect the ceasefire.
Ankara welcomed the ceasefire agreement but has said it will press on with its operations inside Syria.