Actually it wasn’t the US intervention that reversed the militant’s gains.
Sy Gunson

Sorry What? Look at the map of territorial control in Syria when Russians started their involment in Syria and look at the map today. There is literally no gain by Russian backed forces. They captured Palmyra, but than lost it leaving dozens of tanks and vehicles behind. Rebels made some gain against ISIS and large territories were capture by Kurds and this was done thanks to massive Coalition air support. Since 2016 US special forces fight in each large offensive against ISIS. US destroys ISIS infrastructure vehicles and so on US drones take out ISIS leadership. US conducted raids into ISISs territory.

Russians are facused on fighting rebels. Their attacks against ISIS are sporadic and rather PR stunts. Their effort against ISIS are nowhere near US. Russia several times also aided ISIS by bombing rebels on frontline against it.

Russia Blows up ISIS oil convoys ? really? When?