Who are the Kurds?
Between 25 and 35 million Kurds inhabit a mountainous region straddling the borders of Turkey, Iraq, Syria, Iran and Armenia. They make up the fourth-largest ethnic group in the Middle East, but they have never obtained a permanent nation state.
Where do they come from?
The Kurds are one of the indigenous peoples of the Mesopotamian plains and the highlands in what are now south-eastern Turkey, north-eastern Syria, northern Iraq, north-western Iran and south-western Armenia.
Today, they form a distinctive community, united through race, culture and language, even though they have no standard dialect. They also adhere to a number of different religions and creeds, although the majority are Sunni Muslims.
Why don't they have a state?
In the early 20th Century, many Kurds began to consider the creation of a homeland - generally referred to as Kurdistan. After World War One and the defeat of the Ottoman Empire, the victorious Western allies made provision for a Kurdish state in the 1920 Treaty of Sevres.
Such hopes were dashed three years later, however, when the Treaty of Lausanne, which set the boundaries of modern Turkey, made no provision for a Kurdish state and left Kurds with minority status in their respective countries. Over the next 80 years, any move by Kurds to set up an independent state was brutally quashed.
Why were Kurds at the forefront of the fight against IS?
Iraqi Kurdish Peshmerga fighters
n mid-2013, the jihadist group Islamic State (IS) turned its sights on three Kurdish enclaves that bordered territory under its control in northern Syria. It launched repeated attacks that until mid-2014 were repelled by the People's Protection Units (YPG) - the armed wing of the Syrian Kurdish Democratic Union Party (PYD).
An IS advance in northern Iraq in June 2014 also drew that country's Kurds into the conflict. The government of Iraq's autonomous Kurdistan Region sent its Peshmerga forces to areas abandoned by the Iraqi army.
In August 2014, the jihadists launched a surprise offensive and the Peshmerga withdrew from several areas. A number of towns inhabited by religious minorities fell, notably Sinjar, where IS militants killed or captured thousands of Yazidis.
In response, a US-led multinational coalition launched air strikes in northern Iraq and sent military advisers to help the Peshmerga. The YPG and the Kurdistan Workers' Party (PKK), which has fought for Kurdish autonomy in Turkey for three decades and has bases in Iraq, also came to their aid.
In September 2014, IS launched an assault on the enclave around the northern Syrian Kurdish town of Kobane, forcing tens of thousands of people to flee across the nearby Turkish border. Despite the proximity of the fighting, Turkey refused to attack IS positions or allow Turkish Kurds to cross to defend it.
In January 2015, after a battle that left at least 1,600 people dead, Kurdish forces regained control of Kobane.
The Kurds - fighting alongside several local Arab militias under the banner of the Syrian Democratic Forces (SDF) alliance, and helped by US-led coalition air strikes, weapons and advisers - then steadily drove IS out of tens of thousands of square kilometres of territory in north-eastern Syria and established control over a large stretch of the border with Turkey.
In October 2017, SDF fighters captured the de facto IS capital of Raqqa and then advanced south-eastwards into the neighbouring province of Deir al-Zour - the jihadists' last major foothold in Syria.
The last pocket of territory held by IS in Syria - around the village of Baghouz - fell to the SDF in March 2019. The SDF hailed the total elimination of the IS caliphate, but it warned that jihadist sleeper cells remained a great threat to the world.
The SDF was also left to deal with the thousands of suspected IS militants captured during the last two years of the battle, as well as tens of thousands of displaced women and children associated with IS fighters. The US called for the repatriation of foreign nationals among them, but most of their home countries refused to do so.
Now, the Kurds face a military offensive by Turkey, which wants to set up a 32km (20-mile) deep safe zone inside north-eastern Syria to protect its border and resettle up to 2 million Syrian refugees. The SDF says it will defend its territory at all costs and that hard-won gains in the battle against IS are being put at risk.
The Syrian government, which is backed by Russia, also continues to promise to take back control of all of Syria.
Why does Turkey see Kurds as a threat?
There is deep-seated hostility between the Turkish state and the country's Kurds, who constitute 15% to 20% of the population.
Kurds received harsh treatment at the hands of the Turkish authorities for generations. In response to uprisings in the 1920s and 1930s, many Kurds were resettled, Kurdish names and costumes were banned, the use of the Kurdish language was restricted, and even the existence of a Kurdish ethnic identity was denied, with people designated Mountain Turks.
In 1978, Abdullah Ocalan established the PKK
PKK leader Abdullah Ocalan
, which called for an independent state within Turkey. Six years later, the group began an armed struggle. Since then, more than 40,000 people have been killed and hundreds of thousands displaced.
In the 1990s the PKK rolled back on its demand for independence, calling instead for greater cultural and political autonomy, but continued to fight. In 2013, a ceasefire was agreed after secret talks were held.
The ceasefire collapsed in July 2015, after a suicide bombing blamed on IS killed 33 young activists in the mainly Kurdish town of Suruc, near the Syrian border. The PKK accused the authorities of complicity and attacked Turkish soldiers and police. The Turkish government subsequently launched what it called a synchronised war on terror against the PKK and IS.
Since then, several thousand people - including hundreds of civilians - have been killed in clashes in south-eastern Turkey.
Turkey has maintained a military presence in northern Syria since August 2016, when it sent troops and tanks over the border to support a Syrian rebel offensive against IS. Those forces captured the key border town of Jarablus, preventing the YPG-led SDF from seizing the territory itself and linking up with the Kurdish enclave of Afrin to the west.
In 2018, Turkish troops and allied Syrian rebels launched an operation to expel YPG fighters from Afrin. Dozens of civilians were killed and tens of thousands displaced.
Turkey's government says the YPG and the PYD are extensions of the PKK, share its goal of secession through armed struggle, and are terrorist organisations that must be eliminated.