The SDF and coalition launched a major operation to defeat ISIS in a small area along the Euphrates in September 2018. It has been a difficult campaign against the hard core of ISIS supporters. In addition, sand storms and weather slowed the advance last fall. Political considerations, including a Turkish threat to launch operations against the SDF in northern Syria in October, also caused the operation to stop.
In December the US decided to withdraw from Syria, throwing the entire anti-ISIS campaign into question. In addition US President Donald Trump had repeatedly insisted ISIS is defeated. Most recently, on his way back from a meeting in Vietnam he claimed ISIS was 100% defeated.
The situation on the ground has been more complex. Even though ISIS lost almost all its territory, more than 10,000 pro-ISIS supporters are still crammed into a pocket near the town of Baghuz. These were ISIS families and many foreign fighters, including Westerners who had come to the 'caliphate' in 2014 and 2015 with dreams of owning slaves after watching scenes of minorities being murdered by ISIS in Iraq and Syria.