Political map of Russia & the former Soviet Union 28 October 1939 (Molotov–Ribbentrop Pact): On 23 August 1939, even while it was fighting Japan (Battle of Khalkhin Gol) on the Mongolian border, the Soviet Union shocked the world by agreeing to the Molotov–Ribbentrop Pact (Molotov–Ribbentrop Pact) with Nazi Germany (Nazi Germany). Formally, this was a non-aggression pact between two ideological enemies. However a secret clause agreed to the division of eastern Europe. When the Germans invaded Poland in September, the Soviets joined in (Invasion of Poland (1939)) and invaded from the east.