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 Russo-German Molotov–Ribbentrop Pact of August 1939 null and void, and release tens of thousands of Polish prisoners-of-war held in Soviet camps. Pursuant to an agreement between the Polish government-in-exile and Stalin, the Soviets granted "amnesty" to many Polish citizens, from whom a new army (the Polish II Corps) was formed under General Władysław Anders and later evacuated to the Middle East, where Britain faced a dire shortage of military forces. The whereabouts of thousands more Polish officers, however, would remain unknown for two more years, and this would weigh heavily on both Polish–Soviet relations and on Sikorski's fate.