Some background:

- Discrimination -

Concentrated in the north, Kurds make up around 15 percent of Syria's population.

Most are Sunni Muslims but there are some non-Muslim minorities and many Kurds consider themselves secular.

They have suffered decades of marginalisation and oppression by Syria's ruling Baath party and have long pushed for their cultural and political rights.

- Neutrality, then autonomy -

When Syria's conflict erupted in 2011, the Kurdish population generally adopted a position of neutrality.

President Bashar al-Assad at first made conciliatory gestures towards the Kurds, granting citizenship to 300,000 people -- a key demand for half a century.

The Kurds had been stripped of their nationality following a controversial census in 1962.

In 2012, government forces withdrew from Kurdish-majority areas in the north and east, paving the way for Kurds to consolidate control.

They have since established self-rule in many of these zones and have sought to prevent rebels and regime forces from entering those areas.

- 'Federal region' -

In 2013, the Democratic Union Party (PYD) -- the political branch of the powerful People's Protection Units (YPG) -- announced the establishment of a semi-autonomous region.

In 2016, Kurdish authorities unveiled a "federal region" for this territory comprising three cantons: Afrin in Aleppo province, Jazira and Euphrates, which includes parts of Aleppo and Raqa provinces.

The initiative looked like de facto autonomy, provoking hostility from Syria's mainstream opposition forces and neighbouring Turkey.

At the end of 2016, the Kurds gave themselves a "social contract" -- a kind of constitution for their "federal region".

A year later they elected their own municipal councillors.

- Anti-jihadists -

Kurdish fighters have been among the most effective fighting the Islamic State group in Syria, with air support from a US-led coalition.

At the start of 2015, they ousted IS from Kobane on the Turkish border after more than four months of fierce fighting.

The Syrian Democratic Forces (SDF), an alliance of Kurdish forces and local Arab militiamen, was created in October 2015.

Dominated by the Kurdish YPG, it was the main ground force battling the jihadist attempts to set up a "caliphate" in eastern Syria.

In October 2017, they ousted IS from its de facto Syrian capital, Raqa. In March 2019, they drove the jihadists from their last Syrian holdout, the village of Baghouz.

- Turkish ire -

In January 2018, the US-led coalition announced it was working to create in northern Syria a 30,000-strong border force comprised of Kurdish and Arab fighters, around half of whom would be retrained SDF fighters.

But Ankara accuses the YPG of being the Syrian offshoot of the outlawed Kurdistan Workers' Party (PKK) which has waged a three-decade rebellion in Turkey's southeast.

Also in January 2018 Turkey launched an air and ground operation against the YPG-held enclave of Afrin, taking control by mid-March.

- Abandoned by Trump -

In December 2018, US President Donald Trump announced the approximately 2,000 US soldiers deployed in Syria -- fighting alongside the YPG against IS -- would be withdrawn.

In the following days, Turkish President Recep Tayyip Erdogan sent troop reinforcements to the border between Turkey and the Syrian Kurdish regions in preparation for a possible offensive.

But the US maintained a troop presence, after pushback in Washington against Trump's declaration.

On October 6, 2019, the White House announced that US troops would begin pulling back from the border area, making way for the long-threatened Turkish operation.

The retreat began the next day, provoking an angry response by the Kurds and American Republicans, who accused the president of leaving a crucial ally in the fight against IS exposed.