ORIGINS

In March 2011, peaceful protests broke out around Syria as part of the Arab Spring uprisings.

Organisers called on Syrian President Bashar al-Assad to make democratic reforms, but his government responded with violence.

An armed struggle ensued which devolved into a full-blown civil war by 2012.

RUSSIA'S ROLE

Russia, which has long supported Mr Assad's government, has often blocked meaningful international action on Syria by vetoing proposals at the United Nations Security Council.

Moscow changed the course of the war in Mr Assad's favour with a military intervention in 2015.

U.S. INVOLVEMENT

The United States has been reluctant to become too entangled in Syria. But it launched air strikes against the Islamic State in Iraq and Syria after the terrorist group gained a foothold in Syria in 2013.

The US has acted to punish the Assad government for using chemical weapons such as sarin and chlorine gas on Syrian civilians.

The US actions include a cruise missile strike on a Syrian airbase last year after an apparent sarin attack on the town of Khan Sheikhoun killed nearly 100 people. It was the first direct US strike on the Syrian regime in the war.

ASSAD'S KEYREGIONAL ALLIES

One of Mr Assad's key allies is Iran, which needs Syria to move its weapons and proxy militias across the Middle East.

So when Mr Assad seemed threatened, Iran stepped in to support him.

So did Hizbollah, the Lebanese political party and militia that is a close ally of Teheran.

IMPACT ON SYRIANS

Since the start of the war, over 465,000 Syrians have been killed, one million more have been injured, and 12 million - more than half the country's population - have been forced to flee their homes. Over 5.5 million have moved abroad and registered as refugees.

WASHINGTON POST