In 1938 , at the outbreak of World War II , which had a larger population , the Western Allies ( United Kingdom , France , Poland and British Dominions ) or the European Axis ( Germany and Italy )
With an invasion of the Japanese archipelago ( known as Operation Downfall ) imminent , and the Soviet Union 's declaration of war on Japan with the invasion of Manchuria , Japan surrendered , 15 August 1945 .