After ISIS took over large parts of northeastern Syria and northwestern Iraq years ago, the US put together a coalition to combat the terror group. While Western powers brought a lot of air power to the battlefield, they always had problems mustering a viable ground force to reclaim land. The Iraqi and Syrian Kurds proved to be among the most effective anti-ISIS fighters, and Washington relied on them to not only conduct ground offensives but even to call in US airstrikes.
As part of the effort to make the mostly Kurdish SDF more effective, the US and other nations embedded special forces with them. At the end of most of the Syrian combat operations, Turkey began voicing concern about the strong Kurdish presence in northern Syria. Ankara has been in conflict with Kurdish insurgents, both inside and outside Turkish borders, for decades.
The US left forces in Syria to continue the fight against ISIS, but also to prevent Turkey from invading northern Syria and driving out the US-supported Kurdish militias there.
But now, the withdrawal of US troops will open the door for Turkey to push back America's former allies.