The Molotov-Ribbentrop pact, sometimes called the Hitler-Stalin pact, was a non-aggression treaty between Germany and Russia, or more precisely between the Soviet Union and the Third Reich. It was signed in Moscow on August 23, 1939, by the Soviet foreign minister Vyacheslav Molotov and German foreign minister Joachim von Ribbentrop.

On September 1, barely a week after the pact had been signed, the partition of Poland commenced with Germany's invasion. The Soviet Union invaded from the east on September 17, practically concluding a fourth partition of Poland.

The pact caused consternation in the West, both among governments which had most feared such an outcome, and even more so to supporters of communism, many of whom found Soviet dealings with their Nazi ideological enemy incomprehensible. A famous cartoon by David Low from the London Evening Standard of 20 September 1939 has Hitler and Stalin bowing to each other over the corpse of Poland, with Hitler saying "The scum of the Earth, I believe?" and Stalin saying "The bloody assassin of the workers, I presume?".

On September 28th 1939, the three Baltic States were given no choice but to sign a so-called Pact of defence and mutual assistance, which permitted the Soviet Union to station troops in Estonia, Latvia and Lithuania. The same day a supplementary German-Soviet protocol had transferred most of Lithuania from the envisaged German to the Soviet sphere of interest

Finland resisted similar claims, and was invaded by the Soviet Union on November 30. After more than three months of heavy fighting and losses in the ensuing Winter War, the Soviet Union gave up its intended occupation of Finland in exchange for approximately 10% of Finland's territory, most of which was still held by the Finnish army.