Not so much that they let them get a start, as much as ISIS had a useful role to play once they were not an existential threat to Assad. And contrary to their press image, ISIS leaders are fanatics but not berserkers— they were capable of cutting temporary strategic deals.
- ISIS was extremely useful as a tool to attack and undermine U.S. forces and U.S. allies in Iraq and later U.S.- aligned groups in Syria. For Russia, undermining the west is always the first strategic imperative.
- ISIS was a useful rhetorical tool for many years; Russia got a lot of mileage and influence by claiming that they were all that stood between the “U.S. created” ISIS savages and a regional caliphate; in fact they simply effectively kept the stove at a simmer but never a boil.
- ISIS and other jihadi groups are a useful pressure valve to get rid of extremist Muslims inside the Russian federation and make them someone else’s problem, and there is less blowback to killing them abroad than at home.
- Russia and Assad could let them slip off the leash a bit and cause an uptick in migrant flows to Europe which has always been something Russia cannily exploits (Turkey is not innocent here either).
- At one time Russia liked to hold them in reserve as a bargaining chip to get the West to the table. “Let’s go after ISIS together, and put aside all these quarrels over (Russia’s) Near Abroad.”
It’s well documented that Russia and Assad allowed them to operate against common enemies from within the “regime controlled” zone. It’s kind of cliche, but the U.S. vs. Russian view of jihadis sort of reflects the inheritance of Western vs. Eastern Rome (Byzantium). The one likes to bring overwhelming force, wipe them out, completely destroy them, see if you can replace them with something more amenable. The second never wants to fully defeat an enemy because they can always be useful so long as they aren’t an existential threat.