What was the Molotov-Ribbentrop Pact?
The Government of the German Reich and The Government of the Union of Soviet Socialist Republics desirous of strengthening the cause of peace between Germany and the U.S.S.R., and proceeding from the fundamental provisions of the Neutrality Agreement concluded in April, 1926 between Germany and the U.S.S.R. Source: http://www.fordham.edu/halsall/mod/1939pact.html It was signed in Moscow on August 23, 1939 by the Soviet foreign minister Vyacheslav Molotov and German foreign minister Joachim von Ribbentrop. Also called the “Hitler-Stalin” pact.
This agreement was also known as the Nazi-Soviet Non Aggression Pact, which essentially allowed Hitler to invade Poland in 1939, without fearing any repercussion from the USSR. As a matter of fact, it was secretely agreed that Nazi Germany and the Soviet Union would partition Poland, which they did in late 1939. Hitler, of course, had no intention of adhering to this pact, and launched Operation Barbarossa in June 1941, which marked the Nazi invasion of the USSR.