Another little tidbit was that Stalin’s greatest fear was the KPD gaining power in Germany, and ultimately rivaling the Soviet Union for leadership in the worldwide Communist movement. Which is basically why Stalin silently approved of the Nazi roundup of German Communist leaders, like Ernst Thaelmann. Hitler did Stalin’s dirty work for him, in eliminating most of the leadership of the German Communists.
And again in Poland — when Warsaw rose against the Germans during that long retreat, Stalin ordered the Red Army advance stopped so the Germans would be able to destroy the independence forces and take the blame for it.
In 1939, after the Molotov-Ribbentrop Pact, many German Communists who had fled to Moscow after Hitler came to power were arrested and shot. Others were handed back to the Gestapo. A few lucky ones survived and would later run East Germany.