Russian missile strikes and bombing raids in Syria add yet another element to a complex war sparked by anti-government protests that erupted in March 2011.
Fighting groups, both Syrian and foreign, have carved out zones of influence across the country.
- Regime and its allies -
- The Syrian army's 300,000-strong pre-war force has been halved by deaths, defections and frequent draft-dodging. It now controls about a third of Syrian territory, with the rest divided among Islamic State jihadists, Al-Qaeda-linked groups, mainstream rebels and Kurdish forces.
But the regime remains in control of areas where 50 percent of Syria's remaining population live as well as the strategic provinces of Damascus, Latakia and Tartus along the Mediterranean coast, and parts of central Hama and Homs provinces, and Aleppo in the north.
- Some 150,000 to 200,000 men belong to pro-regime militias, mostly the 90,000-strong National Defence Forces, set up in 2012.
A number of militias from Lebanon, Iran, Iraq and Afghanistan have also bolstered the regime's beleaguered forces. The most powerful of these is Lebanon's Shiite movement Hezbollah, which experts say has sent between 5,000 and 8,000 fighters to Syria.
- Russia, a decades-long backer of Syria's regime, launched its first air strikes in Syria on September 30. It had increased its military involvement by building up an airbase in Latakia and sending fighter jets, drones and advanced aerial defence equipment, some of which has been transferred to Syrian troops.
On Wednesday, Moscow said Russian warships in the Caspian Sea had launched cruise missiles on IS positions in Syria and that its air force was providing cover for a Syrian government offensive in Hama.
The Kremlin says its forces have hit more than 100 targets in a week of operations in Syria.
Russia has also dispatched at least 1,700 soldiers to Syria, according to Russian media.
- Iran has remained a staunch ally of President Bashar al-Assad, sending at least 7,000 Revolutionary Guard forces, as well as military advisers and financial aid.
- Rebels and Al-Qaeda -
- Ahrar al-Sham is among the most powerful Islamist rebel groups in Syria. Founded in 2011 and financed by Turkey and Gulf states, according to experts, it has a key presence in Idlib, Aleppo and Damascus provinces.
- Al-Nusra Front is Al-Qaeda's Syria affiliate and the most important jihadist group in the country after its rival, the Islamic State group. Led by Abu Mohammad al-Jolani, it has formed alliances with other rebel groups in Idlib and Aleppo, and is also present in the south and Damascus province. It is listed by Washington as a terrorist group.
Together, Al-Nusra and Ahrar al-Sham lead a key rebel alliance called the Army of Conquest. Since it was founded in 2015, the Army of Conquest has pushed the regime out of Idlib province.
- Jaish al-Islam is the most important rebel group in Damascus province. Led by a Syrian Islamist, Zahran Alloush, the group is based in the Eastern Ghouta region, east of the capital.
- The Southern Front is a coalition of rebel groups in Syria's southern province of Daraa and receives support from the West.
- Islamic State group -
The Islamic State group is the most well-organised, resources-rich and brutally violent non-regime force in Syria. Since 2013, it has seized half of Syria's territory, and it announced a self-styled "caliphate" across Syria and Iraq last year.
Headed by Abu Bakr al-Baghdadi, IS militants are fighting on several fronts: against rival rebels, Al-Nusra, Kurdish groups and the regime.
It has attracted nearly 30,000 foreign fighters to Iraq and Syria since 2011, according to US intelligence estimates.
- The Kurds -
The Kurdish People's Protection Units (YPG) have controlled parts of Syria's north and northeast since the regime unilaterally withdrew its forces from the region in 2012.
The YPG has scored major victories against IS in Kobane, Tal Abyad and Hasakeh province with aerial support from the US-led international coalition fighting the jihadists.
- International coalition -
Since 2014, the United States and Arab nations in a US-led coalition have been carrying out air strikes against IS targets in Syria. It has been unable to eliminate the jihadist group. This year, Turkey and France have joined the coalition strikes in Syria.