US-backed rebels operating out of Jordan have invaded and captured large chunks of southern desert land inside Syria, territory that was nominally under the control of ISIS, although it had little to no ISIS presence, and it does not appear the fighting over it was significant.
The factions, which include a Free Syrian Army (FSA) presence, have long fought over control of the border area around the Jordan-Syria border. They’ve had limited success when they fight with any significant factions, which has kept them mostly confined to the border.
The move into the desert appears likely to have been launched at the behest of Western nations, which have said they believe ISIS intended to use the deserts as an area to flee to if they ended up losing their substantial territory further to the north.
To the extent ISIS had a presence in the area at all, it appears to have centered on important parts of the roads, and a few former Syrian military bases. ISIS is estimated to have withdrawn “hundreds” of fighters into Deir Ezzor Province as a result.