 Complete the following phrase:  Following the German invasion of the Soviet Union Operation Barbarossa in June 1941, Sikorski opened negotiations with the Soviet ambassador to London, Ivan Maisky, to re-establish diplomatic relations between Poland and the Soviet Union, which had been broken off after the Soviet invasion of Poland in September 1939. In December that year, Sikorski went to Moscow with a diplomatic mission. The Polish Government reached an agreement with the Soviet Union the Sikorski-Maisky Pact of 17 August 1941, confirmed by Joseph Stalin in December of that year. Stalin agreed to invalidate the September 1939 Soviet-German partition of Poland, declare the 