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 )
Marked by mass deaths of civilians , including the Holocaust , the Three Alls Policy , the strategic bombing of enemy industrial and / or population centers , and the first use of nuclear weapons in combat , it resulted in an estimated 50 million to 85 million fatalities .