Trump's apparent green light to Ankara — despite opposition from the Pentagon, State Department and much of Congress — marks a stunning abandonment of the SDF, which has said it lost more than 11,000 fighters spearheading the battle against the "Islamic State" (IS) group in Syria.
Why would Turkey want to attack?
Turkey considers the Kurdish YPG, the main component of the SDF, a terrorist group linked to the Kurdistan Workers' Party (PKK), which has fought a nearly four-decade war for Kurdish rights against the Turkish state.
Turkish President Recep Tayyip Erdogan has repeatedly threatened military action in northeast Syria, where the Kurds and local Arab and Christian allies have established a relatively stable, de facto autonomous region during the Syrian civil war.
More so than a military threat, the Syrian Kurdish experiment in "democratic autonomy," based on the libertarian socialist principles of imprisoned PKK leader Abdullah Ocalan, represents an ideological and political challenge to Turkey.
Turkey fears Syrian Kurdish gains will embolden its own Kurdish population and create a PKK-statelet on its border. The combined military, political and territorial strengths of the Syrian Kurds give them a major bargaining chip in any political solution in Syria. They are demanding recognition of Kurdish rights and decentralization of state power. Turkey seeks to prevent this development.
What is Turkey planning?
The scale and size of a potential Turkish operation remain unclear. Turkey wants to create a 32-kilometer-deep, 480-kilometer-long corridor (20 miles deep, 300 miles long) inside Syria along the border to protect its security.
Turkey says it plans to resettle nearly 1 million of its 3.6 million Syrian refugees, who hail from other parts of Syria, inside the "safe zone." In reality, a buffer zone may only extend several kilometers inside Syria and be formed around several pockets, and the operation conducted in stages.
With the support of the Turkish military, Turkey plans to use a motley group of its Syrian rebel allies to do much of the ground fighting and holding of territory.
The extent of the operation may be hampered after the US military said it was closing Syrian airspace to Turkish warplanes and US officials warned Turkey about any major operation.
Syrian Kurds are warning of ethnic cleansing and demographic engineering of areas along the border.
There are an estimated 1.8 million Kurds in Syria, about half of whom live within Turkey's proposed buffer zone. SDF-controlled areas are believed to have around 1.5 million Arabs and tens of thousands of Christians.
In a statement, the SDF said it was "determined to defend our land at all costs." The SDF has an estimated 60,000 fighters.
The landscape of northeast Syria is open plains, which would make it difficult for the lightly armed SDF to resist NATO's second-largest army.
A Turkish military incursion into northeast Syria would likely send hundreds of thousands of civilians fleeing into SDF-controlled areas further south and into neighboring Iraqi Kurdistan.
Turkey's plans would also exacerbate ethnic tensions for years to come and leave Ankara battling a long-running Kurdish insurgency.
The SDF warns that a fight with Turkey would force it to redeploy fighters and distract from securing the gains of its fight against IS. It has also warned that around 11,000 IS fighters in SDF prisons could flee in any chaos.
US support for the SDF has been a major source of tension with NATO ally Turkey.
In December 2018, Trump announced that the roughly 2,000 US troops in Syria would be withdrawn because IS had been "defeated." Under pressure from the Pentagon, Congress and European allies, he then reversed course.
Even Turkey, caught off guard by an unexpected US intention to retreat, was cautious about a hasty US withdrawal. But the announcement paved the way for months of talks between Turkish and US officials that led in August of this year to an agreement on a joint security mechanism to establish a "safe zone" in northeastern Syria.
Despite the agreement, both the US and Turkey remained at odds over the extent and long-term nature of the "safe zone."
The agreement led to joint US-Turkish patrols inside a limited buffer zone to address Turkey's concerns over the Syrian Kurds and the SDF dismantling its defenses along the border and pulling back heavy weaponry.
The agreement, which had buy-in from the SDF, was meant to buy time and appease Turkey.
Unsurprisingly, the SDF feels betrayed by Trump's announcement to pull US troops from the border.
"We are not expecting the US to protect northeast Syria," SDF spokesman Mustafa Bali said on Twitter on Monday. "But people here are owed an explanation regarding the security mechanism deal, destruction of fortifications and failure of US to fulfill their commitments."
"US forces did not fulfill their responsibilities and began withdrawing from the border, leaving the area to turn into a war zone. But SDF is determined to defend northeast Syria at all costs."
What has Turkey done in the past?
Turkey has carried out two previous military operations inside Syria to thwart Kurdish ambitions.
In 2016, the Turkish military and its rebel allies launched Operation Euphrates Shield. The cross-border invasion cleared IS from an area spanning Jarabulus on the Euphrates River to Azaz in the west near the Kurdish-held enclave of Afrin.
The military operation was intended to block the Syrian Kurds from linking territories under their control in the northeast with Afrin. Unlike northeast Syria, the Turkish-controlled Euphrates Shield zone is predominately Arab. Turkey has resettled Syrian refugees in the area.
In 2018, Turkey launched an operation into Afrin that displaced roughly half of the enclave's population of nearly 300,000. The YPG continues a low-level insurgency against the Turkish military and its Syrian rebel allies.
Rights groups have accused Turkish-backed forces of gross human rights violations, including forcible displacement, confiscation of property, pillaging, arbitrary arrest, torture, kidnapping and extortion. Turkey has moved the families of rebel fighters and other refugees into Afrin.