Why is there a war in Syria?

External powers have also been accused of fostering sectarianism in what was a broadly secular state, pitching the country's Sunni majority against the president's Shia Alawite sect. Such divisions have encouraged both sides to commit atrocities that have not only caused loss of life but also torn apart communities, hardened positions and dimmed hopes for a political settlement.

Jihadist groups have also seized on the divisions, and their rise has added a further dimension to the war. Hayat Tahrir al-Sham, an alliance formed by what was once the al-Qaeda-affiliated al-Nusra Front, controls large parts of the north-west.

Meanwhile, the Islamic State (IS) group seized control of large swathes of north-eastern Syria. It now controls only a few isolated pockets of territory after being driven from its urban strongholds by Russian-backed government forces, Turkish-backed rebel brigades, and a Kurdish militia alliance supported by the US.

Thousands of Shia militiamen from Iran, Lebanon, Iraq, Afghanistan and Yemen are fighting alongside the Syrian army, in order, they say, to protect Shia holy sites.

Why are so many outside powers involved?

Russia, for whom President Assad's survival is critical to maintaining its interests in Syria, launched an air campaign in September 2015 with the aim of "stabilising" the government. Moscow stressed that it would target only "terrorists", but activists said its strikes repeatedly hit Western-backed rebel groups and civilian areas.

The intervention has turned the tide of the war in Mr Assad's favour. Intense Russian air and missile strikes were decisive in the battle for rebel-held eastern Aleppo in late 2016, while Russian special forces and mercenaries helped break the long-running IS siege of Deir al-Zour in September 2017.

Two months later, President Vladimir Putin ordered a partial withdrawal of Russian forces, but they have continued to conduct air strikes across the country.

Shia power Iran is believed to be spending billions of dollars a year to bolster the Alawite-dominated government, providing military advisers and subsidised weapons, as well as lines of credit and oil transfers. It is also widely reported to have deployed hundreds of combat troops in Syria.

Mr Assad is Iran's closest Arab ally and Syria is the main transit point for Iranian weapons shipments to the Lebanese Shia Islamist movement Hezbollah, which has sent thousands of fighters to support government forces.

Israel has been so concerned by Hezbollah's acquisition of advanced weaponry and what it calls Iran's "entrenchment" in Syria that it has carried out dozens of air strikes seeking to thwart them.

The US, which says President Assad is responsible for widespread atrocities, supports the opposition and once provided military assistance to "moderate" rebels. It has also conducted air strikes on IS in Syria since September 2014, but has only targeted pro-government forces on a few occasions.

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