Your challenge, with a POD no earlier than 1920, is to have a weakened Soviet Union facing Nazi Germany, to the point that a Nazi victory in the east is realistic or even likely.
In November, 1932, Nadezhda Sergeevna Alliluyeva shoots Stalin at their villa, and then commits suicide. The USSR is in the midst of the 1932-33 famine, and the Party has already begun show trials. There are a series of confrontations between the military, the Party, and various factions within each, which serve to further weaken the state and party apparatus. Molotov and Voroshilov win out over Kamenev and Zinoviev, but the conflict leaves the two "victors" in an uneasy partnership, which weakens the re-armament and economic development programs. In a bid to strengthen their respective positions, Molotov and Voroshilov take advantage of the "mysterious" murder of Kirov in 1934 to stage a series of show trials of their own, which are used to dismiss and murder much the same range of individuals who were targeted historically. This allows Molotov and Voroshilov to consolidate power, which in turn leads to Molotov negotiating with Hitler. In 1939, the USSR participates in the carve-up of Poland, Romania, and the Baltic states; in 1941, Hitler turns east...