A new ISIS attack on Syrian government forces today will deepen these beliefs.
Since November 2017 the U.S. and its proxy forces in north-east Syria did absolutely nothing against ISIS in east Syria north of the Euphrates. U.S. air strike were stopped and ISIS's territorial hold did not change one bit.
In February local tribal forces aligned with the Syrian government crossed the Euphrates from south to north in order to attack the ISIS pocket and to take control of an oilfield. The U.S. claimed that its Kurdish SDF proxy forces were attacked by the Syrian government aligned group. Curiously no one on the side of the U.S. and its proxies was hurt at all. Soon a large number of U.S. air support assets arrived and bombed the Syrian group to smithereens.
ISIS in the northeastern pocket is the justification for the continuing U.S. occupation. But when Syrian government forces attacked those ISIS forces the U.S. claimed that only its forces were there. On June 6, six months after the U.S. had stopped attacking ISIS, U.S. Secretary of Defense Mattis finally announced that U.S. proxies forces had again taken up the fight: