The Soviet Union's participation in World War II began with the Battles of Khalkhin Gol against Japan, in Mongolia between May–August, 1939. Later that year, protected with the Molotov–Ribbentrop Pact, it invaded eastern Poland about three weeks after the Germans invaded the west of the country. During the next eleven months the Soviets occupied and annexed the Baltic states. The Soviet Union supported Germany in the war effort against Western Europe through the 1939 German–Soviet Commercial Agreement and larger 1940 German–Soviet Commercial Agreement with supplies of raw materials that were otherwise blocked by the British naval blockade. Following Finland's refusal of Soviet demands for military bases and a territorial swap, the Soviet Union invaded on 30 November 1939, in the Winter War. The Soviet Union also annexed Bessarabia, leading Romania to ally with Germany.