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 )
Mussolini was aware that Italy did not have the military capacity to carry out a long war with France or the United Kingdom and waited until France was on the verge of imminent collapse and surrender from the German invasion before declaring war on France and the United Kingdom on 10 June 1940 , on the assumption that the war would be short - lived following France 's collapse .