The relentless U.S. opposition to ISIS.

From an interesting account of the deal between the Coalition, the SDF, and ISIS allowing ISIS fighters and their families to flee Raqqah:

Along the route, many people we spoke to said they heard coalition aircraft, sometimes drones, following the convoy.

From the cab of his truck, Abu Fawzi watched as a coalition warplane flew overhead, dropping illumination flares, which lit up the convoy and the road ahead.

When the last of the convoy were about to cross, a US jet flew very low and deployed flares to light up the area. IS fighters [were very scared].”^[1]

Notes
^[1] "Raqqa’s dirty secret." By Quentin Sommerville and Riam Dalati, BBC, 11/13/17.