What was the USSR doing between September 1939 and June 1941? It was allied with nazi Germany, of course. Historian Roger Moorhouse, author of books including The Devils' Alliance, describes the lasting impact of the Molotov-Ribbentrop pact, and the strategic factors that ended it.

Roger Moorhouse

The Devils' Alliance

Roger Moorhouse's books: https://www.rogermoorhouse.com/books

Map: The division of eastern Europe according to the secret protocols of the pact