I watched the AI Soviets take, via focus or justifying, Iran, Iraq, Afghanistan, and Turkey, then puppet them all, (in no particular order). I think Turkey was first. They annexed istabul / the straits, and puppeted the rest. Which is really smart because they will occupy the rest of eastern Europe just by annexing the European border portion.
This was because Germany I guess was afraid to declare. The Soviets were afraid to declare on Germany. Or maybe there was a massive defensive build up on both sides by 1942-43.
But it worked out really well for them because it basically added a third front in Turkey around Istanbul / Bulgaria, and also allowed the Soviets to expand into North Africa when the war came. Everything that Italy steals in 1939-1940 can be yours later on.
Granted this isn't always possible because the German AI might declare in 41.
But I think it worked ok because the Germans went absolutely nowhere.
Obviously theres a lot of USSR specific stuff, the purge, etc.