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 )
Following the Potsdam Declaration by the Allies on 26 July 1945 , the United States dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki on 6 August and 9 August respectively .