Which countries have been involved in the war and why?

A stunning advance by Syrian rebels ended Bashar al-Assad’s decades-long rule, with opposition forces taking the capital and forcing the president to flee on 8 December.

The overthrow followed a 13-year civil war, which started after Assad crushed pro-democracy protests, killing more than half a million people and displacing millions more, and embroiled international powers and their proxies.

The world is now watching to see how Syria’s political landscape shapes up after the overthrow of the Assad family’s half-century rule.

Those with a vested interest in the conflict and the future of the country include, on one side, Russia and Iran – which backed Assad – and on the other, the US and Turkey, which supported different rebel groups.

Here we explore how those countries, along with Israel, have played a role in Syria – and could continue to do so.

During Syria’s civil war, Turkey has supported opposition forces – primarily the Syrian National Army (SNA) – by providing arms, military and political support.

Syria’s northern neighbour has mostly been concerned with using rebels to contain the Kurdish YPG militia, which Turkey accuses of being an extension of a domestically banned Kurdish rebel group. Turkey also wants the roughly three million Syrian refugees living in its country to return home.

The YPG is the biggest militia in another rebel group, the US-backed and Kurdish-led Syrian Democratic Forces (SDF) alliance.

During the war, Turkish troops and allied rebels seized stretches of territory from these groups along Syria’s northern border.

Turkey has also been politically involved. In 2020, Turkey and Russia brokered a ceasefire to halt a push by the government to retake Idlib, the rebels’ stronghold in the north-west.

Idlib has been administered since 2017 under a so-called government by Islamist militant group Hayat Tahrir al-Sham (HTS), which led the rebels that eventually overthrew Assad.

Many believe the offensive could not have happened without Turkey’s blessing. Turkey has denied backing HTS.

Meanwhile, conflict in Syria’s north continues: As Assad fell, the SNA launched a separate assault on areas held by the SDF.

Russia already had a decades-long relationship with the Assad government, and had military bases there before the civil war.

In 2015, Russia launched an air campaign and sent thousands of troops in support of the Assad regime.

In return, Russia received 49-year leases on an air base and naval base, which provided crucial hubs in the eastern Mediterranean for transferring military contractors in and out of Africa.

But fighting a war in Ukraine since 2022 preoccupied Assad’s ally, contributing to the Syrian military’s swift defeat to rebel groups in late November and early December.

Assad and his family were granted asylum in Moscow after fleeing Damascus, Russian media reported.

After Syria’s pro-democracy protests in 2011 were met with force, then US-president Barack Obama criticised the Assad government – but the US only became involved militarily in order to combat the Islamic State (IS).

A US-led global coalition has carried out air strikes and deployed special forces since 2014 to help the Kurdish-led rebel alliance SDF capture territory once held by IS militants in the north-east.

After the Assad government fell, the US government said it conducted dozens of air strikes against IS camps and operatives in central Syria to ensure IS could not take advantage of the unstable situation.

However, President-elect Donald Trump, who takes office in January, said Syria is a “mess” the US should stay out of. When Trump was president in 2019, he withdrew US troops from Syria, a move his officials gradually rowed back.

The US currently has around 900 troops in Syria.

Iran and Syria have been allies since Iran’s Islamic Revolution in 1979. Syria backed Iran during the Iran-Iraq war in the 1980s.

During the Syrian civil war, Iran is believed to have deployed hundreds of troops and spent billions of dollars to help Assad.

Thousands of Shia Muslim fighters armed, trained and financed by Iran – mostly from the Lebanon-based Hezbollah movement, but also from Iraq, Afghanistan and Yemen – have also fought alongside the Syrian army.

But similar to Russia, Hezbollah has been weakened by conflict with Israel in Lebanon, likely hastening the downfall of the Syrian military.

Israel shares a border with Syria. In the 1967 Six-Day War, Israel seized the Golan Heights, about 60km (40 miles) south of Damascus, from Syria, before annexing it in 1981. The annexation is not recognised by the UN and many other countries.

Israel has conducted air strikes against Iran-linked targets in Syria during the war, although it rarely acknowledges such strikes.

Since rebels overthrew Assad, Israel has carried out hundreds of attacks across Syria. Targets include Syria’s military infrastructure, naval fleet, and weapons production sites.

Israel said it is acting to stop weapons falling “into the hands of extremists”.

Israeli forces have also seized the demilitarised buffer zone in the Golan Heights and may have strayed into nearby Syrian territory.

BBC Verify geolocated an image of an IDF soldier standing just over half a kilometre beyond the buffer zone, inside Syria on a hillside near the village of Kwdana.