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 )
Pursuant to a then - secret provision of its non-aggression Molotov - Ribbentrop Pact , the Soviet Union joined with Germany on September 17 , 1939 , to conquer Poland and to divide Eastern Europe .