Political map of East Asia and the Western Pacific 31 August 1939 (Nomonhan Incident): A second Soviet-Japanese dispute broke out on the Mongolian border in May 1939 (Battles of Khalkhin Gol), culminating in the Soviet defeat of Japanese forces at Nomonhan in August. At the same time, the Soviets signed the Molotov-Ribbentrop Pact, a non-aggression treaty with Japan's erstwhile ally Germany.