Over the past years, US troops have supported Kurdish fighters as they battled radical "Islamic State" (IS) militants to take back control of large areas of northern Syria. In what was seen as a surprising turnaround, US President Donald Trump announced in early October that he was withdrawing US troops from the region's border with Turkey. This pullback left a vacuum for others to act and react.

Trump's troop withdrawal was a de-facto go-ahead for Turkey to launch an offensive into northeast Syria. The region is home to a largely autonomous Kurdish population and Kurdish militants known as the YPG, who are tied to an outlawed Kurdish party in Turkey. Turkey, who has faced a Kurdish insurgency, sees the Syrian Kurds as a threat to its security, hence the military action.

The YPG was one of the US' main allies in the fight to drive out IS from north Syria, but since October it has been fighting the Turkish forces that crossed into Syria. The YPG lacks strong air capabilities and defenses, putting it at a decided disadvantage in comparison to the Turkish army.

The YPG is the largest component of the Syrian Democratic Forces (SDF), which also includes Arab and Christian militias. The SDF, which fought IS, controls northeastern Syria and feels betrayed by the US pullback. It is now fighting Turkish troops and their allies. It has warned that the Turkish offensive could distract from making sure IS fighters do not renew their strength in Syria.

The relationship between Syrian President Bashar Assad's troops and the SDF is a tricky one that shifts between cooperation, live-and-let-live and skirmishes, depending on the current situation. After Turkey launched its offensive, the Kurds struck a deal with the government that saw Syrian troops mobilized to fight the Turkish forces, allowing them to enter a region they had ceded to the SDF.

Russia has consistently backed the government of Syrian President Assad (L, with Putin in 2018) and assisted its forces. After US troops pulled out of the Kurdish areas, Russia moved its troops in to act as a buffer for Syrian government forces advancing towards the Turkish army. Moscow wants Syria to remain united and has accused the US of creating parallel structures in the Kurdish region.

Turkey also has allies among Syrian fighters. The Syrian National Army (SNA), also known as the Free Syrian Army, is a Syrian rebel group that has fought against the SDF and Assad's government. Backed by Turkey, SNA fighters took part in previous Turkish offensives against Kurdish militias inside Syria. Currently, thousands of SNA fighters are fighting the YPG alongside Turkish forces.

One possible future actor is IS. While it was essentially defeated in March 2019, tens of thousands of its fighters and their families remain in prisons or guarded camps in the Kurdish area of the country. Nearly a thousand alone have already escaped from a camp that was caught in the fighting between Kurdish militias and Turkish forces. Should the situation grow more unstable, IS could regroup.