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 )
The Empire of Japan aimed to dominate East Asia and was already at war with the Republic of China in 1937 , but the world war is generally said to have begun on 1 September 1939 with the invasion of Poland by Germany and subsequent declarations of war on Germany by France and the United Kingdom .