During the Second World War, Sikorski became Prime Minister of the Polish government-in-exile, Commander-in-Chief of the Polish Armed Forces, and a vigorous advocate of the Polish cause in the diplomatic sphere. He supported the reestablishment of diplomatic relations between Poland and the Soviet Union, which had been severed after the Soviet pact with Germany and the 1939 invasion of Poland—however, Soviet leader Joseph Stalin broke off Soviet-Polish diplomatic relations in April 1943 following Sikorski's request that the International Red Cross investigate the Katyń Forest massacre.