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 )
Though the Allies ' economic and population advantages were largely mitigated during the initial rapid blitzkrieg attacks of Germany and Japan , they became the decisive factor by 1942 , after the United States and Soviet Union joined the Allies , as the war largely settled into one of attrition .