Here is an overview of the powers involved:

- Turkey: Syria's neighbour -

The offensive Turkey launched on Wednesday is its third into Syria since the conflict erupted, targeting principally the Kurdish People's Protection Units (YPG) militia.

The first, from August 2016 to March 2017, was aimed at YPG fighters as well as the Islamic State (IS) jihadist group.

Syrian rebels backed by Turkey captured several areas from IS's grip, including the jihadist bastion of Al-Bab.

The intervention also allowed Turkey to create a buffer between Kurdish-held areas in northern Syria.

Its second offensive, from January to March 2018, exclusively targeted the YPG and ousted them from their enclave of Afrin.

Ankara says the militia is a "terrorist" offshoot of the outlawed Kurdistan Workers' Party (PKK), which has been waging an insurgency in Turkey for three decades.

But the West for several years backed the YPG, a crucial force in the defeat of IS.

- The US: Obama to Trump -

The US-led intervention began in 2014, under Barack Obama's presidency, after IS fighters seized large areas of Iraq and Syria.

Washington formed a coalition of more than 70 countries and several began bombarding IS positions in September that year.

The dominant contributor in the coalition, Washington deployed 2,000 soldiers -- mainly from the special forces -- and mobilised significant air and naval power.

In 2018, President Donald Trump announced shock plans for an immediate pullout of US troops from Syria, specifying months later that about 400 would remain on the ground "for a period of time".

The White House announcement on October 6, 2019 that US "special operators" would be moved back from the Syrian border cleared the way for the long-threatened third Turkish operation, which began just three days later.

- Russia: Damascus's main backer -

In 2015, Russia began air strikes in Syria to back President Bashar al-Assad's regime.

It marked a turning point in the war, enabling pro-regime forces to wrest back lost territory in a series of stunning victories.

The campaign has exacted a huge toll, both in terms of casualties and sheer destruction, including of large swathes of the ancient city of Aleppo.

Moscow has two military bases in Syria: the aerodrome in Hmeimim in the northwest and the port of Tartous, further south.

More than 63,000 Russian military personnel have seen combat in Syria, Moscow says, with 3,000 Russian troops there in October 2019, along with planes, helicopters, warships and submarines.

- Iran: Assad's ally -

Since the start of the conflict, Iran has offered crucial military and financial support to the regime.

Tehran denies deploying regular troops to Syria but says that its elite Revolutionary Guard Corps has "military advisers" with pro-regime forces.

Thousands of "volunteers" from Iran, Afghanistan or Pakistan are also on the ground, it says.

Iran's ally, the powerful Lebanese Shiite movement Hezbollah, officially announced its military backing for Damascus in 2013, though experts say its fighters had been deployed on the ground long before.

Hezbollah said in July 2019 that it had reduced its fighting force in Syria, which experts said had numbered between 5,000 and 8,000.

- Israel: Iran's arch enemy -

Israel and Syria are officially at war and the conflict has escalated tensions between them due to the involvement of two of Israel's arch enemies, Hezbollah and Iran.

Israel has carried out hundreds of strikes on Syrian territory against targets it says belong to Iran and Hezbollah, but rarely claims responsibility.