Why is Turkey planning an assault?
One main reason: Turkey considers the biggest militia in the Kurdish-led alliance a terrorist group. It says it is an extension of a Kurdish rebel group fighting in Turkey.
Turkish leaders want a 32km (20-mile) deep "safe zone" along the Syrian side of the border clear of Kurdish fighters. They also hope to resettle up to 2 million Syrian refugees currently living in Turkey there.
The Kurdish-led alliance says it will defend its territory and that the US is "leaving the area to turn into a war zone" and risking the re-emergence of IS.
Turkish leaders view the YPG as an extension of the banned Kurdistan Workers' Party (PKK), which has fought for Kurdish autonomy in Turkey for three decades.
The YPG dominates an alliance of Kurdish and Arab militias called the Syrian Democratic Forces (SDF), which has driven IS out of a third of Syria over the past four years with the help of air strikes by a US-led coalition.
Turkey has condemned the US for supporting the YPG and has carried out two cross-border offensives against the Kurdish forces.
That December, with IS close to defeat, President Donald Trump said the US would begin withdrawing its troops from Syria. When commanders and allies expressed concern about the fate of the Kurds, he vowed to "devastate Turkey economically" it attacked them and proposed a "20-mile safe zone" along the border.
Mr Trump later suspended the withdrawal, but Turkish President Recep Erdogan continued pressing for a safe zone.
In August, the US and Turkey agreed to establish one together. Kurdish officials expressed support and the YPG dismantled border fortifications.
But two months later, Mr Trump decided to let Turkish troops set up the safe zone alone.
Mr Erdogan is confident his proposed 480km-long corridor will ensure Turkey's border security and become home to between 1 and 2 million Syrian refugees.
Turkey will also take responsibility for all captured IS militants there, according to the White House.
The SDF says it has been "stabbed in the back" by the US, and warns that a Turkish offensive will create a "permanent warzone" and reverse the defeat of IS.