Is that a clickroll?
Anyway, this is actually easier than you might think. Russian and British troops occupied Persia during WW1, and again in WW2. The best POD is probably during the early 1920s- a Persian SSR was set up in Rasht with Red Army assistance, but when that support was withdrawn in 1921- after a treaty between the Soviets and the UK- it collapsed as Reza Pahlavi took power. Having it survive would be tricky, but not impossible- perhaps it is more successful and the Soviets keep supporting it until it takes control of the whole country rather than pulling out in 1921.