All the mentioned parts of land were negotiated in the Ribbentrop pact. So the USSR was allowed to take them whenever felt the time was right. Hitler agreed to Stalin´s requests in 1939 because he needed the pact in Aug 1939 desperately, and believed everything would change one day back to normal, i.e. Barbarossa plan.

I am not sure why Stalin really made the pact with the Yugoslav´s but the Slav ties are always very close ( if they meant anything to Stalin ever really ).

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