US President Donald Trump has been widely criticised over a decision to abandon allied Kurdish-led forces — who played a crucial role in defeating the Islamic State (IS) in Iraq and Syria — effectively green-lighting a Turkish invasion of Syria's largely Kurdish north.

Key points: A US withdrawal will leave their Kurdish allies open to a full-scale Turkish invasion

A US withdrawal will leave their Kurdish allies open to a full-scale Turkish invasion A Turkish incursion "could have major humanitarian consequences", experts say

A Turkish incursion "could have major humanitarian consequences", experts say Turkey plans to establish a 32-kilometre "safe zone" along the Syrian side of the border

The White House announced late on Sunday (local time) that Turkey would soon move forward with a long-planned military operation into north-east Syria to create a "safe zone" along its borders, adding that US forces would not be involved.

This was followed by mixed signals from Mr Trump, who, after declaring US troops stationed in the area would step aside for the expected Turkish attack, then threatened to destroy the Turks' economy if they went too far.

Mr Trump's decision to withdraw support for Kurdish allies was met with criticism even from among members of his own party, who called the move "a catastrophic mistake", and urged Mr Trump to reconsider his decision.

Syria's Kurds are furious over an abrupt US troop pull-back that exposes them to a threatened attack by their nemesis, Turkey. ( AP: Bram Janssen )

"If the President sticks with this retreat, he needs to know that this bad decision will likely result in the slaughter of allies who fought with us, including women and children," said Ben Sasse, a Republican member of the Senate Intelligence Committee.

But Mr Trump defended his position via Twitter, saying although the Kurds "fought with us", they were paid in money and equipment to do so.

"It is time for us to get out of these ridiculous endless wars … and bring our soldiers home," Mr Trump tweeted.

Here we look at why Turkey is so eager to defeat Syrian Kurdish forces, the controversies around Mr Trump's decision, and what it might mean for the future of the Kurds.

Why does Turkey want to attack Syria's Kurdish forces?

Donald Trump speaks to Turkey's President Recep Tayyip Erdogan. ( Reuters: Joshua Roberts )

To understand why Turkey would want to crush a minority group in a foreign country, it's important to understand who the Kurds are and where they came from.

In a nutshell, the Kurds are the world's largest ethnic minority without a country of their own — there's roughly 35 million Kurds living across the borders of Iraq, Syria, Iran and Turkey — the latter of which has the highest Kurdish population.

The crux of the Kurdish crisis dates back to the post-World-War-I era after the fall of the Ottoman Empire when the French and British drew up a map for the Middle East that failed to identify various ethnicities and groups living across the region, hence splitting the Kurds across four different countries.

For decades they've fought for the creation of Kurdistan, while faced with repression from their respective governments, leading to different factions and Kurdish groups across the region — some militant, others not — such as the PKK in Turkey, the YPG in Syria and the Peshmerga in Iraq, among others.

When Islamic State (IS) plundered throughout the Middle East in 2014, ripping apart Western borders across Syria and Iraq, the Kurds coincidentally found themselves on the same battlefield, and were by far the most successful group at holding back the Islamists as they were unified across the Iraq, Syria and Turkish borders in extremely difficult-to-navigate terrain.

Map of the Middle East showing Kurdish distribution. ( Reuters )

Later, backed by the United States' fight against IS, the battle pulled various Kurdish forces together — the move and success by the Kurds inspired renewed talk that Kurdistan in some shape or form could be realised one day if they helped defeat IS.

However, for countries like Turkey, who have long had militant struggles with their own Kurdish population in the south-east, the situation presented a more controversial and complicated prospect.

The Kurdish Workers Party, known widely by the acronym PKK, took up arms in Turkey in 1984 with the aim of creating an autonomous Kurdish state by force — many allied nations including the US and Australia, have designated the PKK a terrorist group.

While Syrian Kurdish leaders say they are separate to the PKK while also having alliances with US forces against IS, Turkey very much sees them as an extension of the PKK, and the idea of a unified, strengthened, international Kurdish militant force along Turkey's south-eastern borders is a highly undesirable situation.

From late 2015 until today, this has led to Turkey's long-held battles with their own Kurdish population spilling over onto Syria's Kurds during the fight against Islamic State.

Turkey's president Recep Tayyip Erdogan has maintained he would "vanquish" Kurdish forces that he says are "terrorists" and a threat to Turkish security.

What's behind the criticism of Trump's decision?

Donald Trump is under pressure to weigh this decision carefully. ( AFP: Delil Souleiman )

United States troops have worked alongside Kurdish forces on the ground and provided crucial support for years in the fight against Islamic State.

Many Kurds across the region were hoping that if and once victorious against the brutal Islamists, they may have hope for establishing an independent country, which was a key motivator in their involvement in the battle and alliance with the US.

However, just like the post-WWI era, the Kurds repeatedly were handed the short end of the stick.

For example, in 2017, following the defeat of IS in Iraq, Iraqi Kurds held an independence referendum which more than 90 per cent voted in favour of — Western allies remained against holding the vote, and Iraq responded by delegitimising it, resulting in their leader standing down.

Then, in 2018, after the US announced the "defeat" of IS in Syria, Mr Trump tabled plans to begin withdrawing US troops — but after accusations of "abandoning" his Kurdish allies, he suspended that withdrawal.

In August 2019, the US and Turkey agreed to establish a joint buffer zone along the Turkish-Syrian border, which Kurdish officials expressed support for and proceeded to dismantle border fortifications amid assurances Turkey would not invade.

In the time since, Mr Trump announced the US would be withdrawing from northern Syria, while backing Turkey's war against the Kurds with statements referring to them as "natural enemies".

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"We interject ourselves into tribal wars and they're not the kind of thing that you settle the way we would like to see it settled," Mr Trump said.

"It doesn't work that way. Hopefully, that will all be very strong and strongly done."

Turkey plans to establish a 32-kilometre "safe zone" along the Syrian side of the border, clear of Kurdish fighters. They also hope to use the area to resettle some of the 3.6 million Syrian refugees currently living in Turkey.

What could a US withdrawal mean for the Kurds?

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 41 seconds 41 s Mr Trump said it was time for the US to withdraw from "ridiculous endless wars".

If the US withdraws, they will leave their Kurdish allies open to a full-scale Turkish invasion.

Republicans and Democrats have warned that allowing the Turkish attack would send a troubling message to American allies across the globe.

Refugees International president Eric Schwartz said a Turkish incursion "could have major humanitarian consequences".

"It could open new fronts in the conflict and displace hundreds of thousands of civilians across an area already in the grip of a humanitarian crisis," he said, adding it would also likely force international aid groups to evacuate "just when they are most needed".

Mr Schwartz labelled Mr Erdogan's plans to return up to a million Syrian refugees to a 'safe zone' "shockingly irresponsible and will put lives at grave risk".

"If Turkey is allowed to attack, the Syrian Kurds could lose everything," said Wladimir van Wilgenburg, analyst and author of The Kurds of Northern Syria.

"Turkey is planning to settle 2-3 million Syrians in the northeast. If this happens after Turkey occupies the northeast of Syria, that would mean there would be no more future for Kurds in Syria."

The Kurds say their ethnic identity, language and culture have been suppressed. ( ABC News: Tracey Shelton )

Mr Van Wilgenburg said the SDF had been removing defence tunnels and fortifications based on promises from US officials that there would be no attack.

"But then after one call of Erdogan, Trump changed his mind," he told the ABC.

Mr Van Wilgenburg said while the Kurds would be open to dialogue with Turkey, they were also ready to defend themselves against a Turkish attack.

"They are already removing forces from areas bordering with the Syrian Government, such as in the Omar oil field," he said.

They may also turn to Syrian President Bashar al Assad to cut a deal.

Kurdish forces have had a precarious relationship with Syrian forces since the beginning of the conflict in 2011, but Mr Van Wilgenburg warned Mr Assad would be unlikely to make concessions.

"That would be a very hard and difficult bargain," he told the ABC.

But Mr Trump has changed his mind in the past about withdrawing support for the Kurds, and he is under pressure to weigh this decision carefully.

An offensive in Syria could also have dire consequences for Turkey.

Senator Sasse warned: "Before Turkey butchers innocent Kurds, Mr Erdogan should carefully consider his privileged status as a NATO member."