Because of the Nazi-Soviet Pact, signed in 1939, the Communist Party was briefly tolerated in the early stages of the occupation. Coinciding with the invasion of the Soviet Union in June 1941 however, the Germans rounded up a large number of Communists (identified in police dossiers compiled before the war) in an operation codenamed "Summer Solstice" ("Sommersonnenwende"). In September 1942, the Germans arrested over 400 workers which they feared were plotting a large-scale strike action.