World War II was a total war in the sense that the countries involved employed all their resources in order to help the war effort. Just as this war spread over a greater geographical extent than other wars, its nature, too, became all-encompassing. Unlike previous wars, the scale of involvement spread far beyond just the use of armies and navies. It was not just the actual fighting that mattered, but the continuous manufacturing of arms and equipment to help the war effort. This was a highly mechanized war, more so than previous conflicts, and consequently the demand for modern, highly specialized equipment - fighter planes, guns, bombs, and so on - was huge. Additionally, in countries like Britain, food and clothes were rationed, and many people helped out in volunteer organizations all over the country to boost the war effort. The war was viewed very much, then, as a national affair. This feeling was increased by the use of modern communications, principally radio broadcasts which kept the people updated on what was happening and, just as importantly, endeavoured to boost the national morale by constant appeals to patriotism and collective strength.
Of course, there was a whole other and much grimmer side to this unprecedented scale of civilian involvement in the form of frequent air-raids on major cities. Civilians became a widespread target in this war. There was the London Blitz, the destruction of Dresden, and, finally, at the end stage of the conflict, the dropping of the first nuclear bombs on the Japanese cities of Hiroshima and Nagasaki.
The definition of a total war is a war that is unrestricted in terms of the weapons used and the territory fought on and the people involved. It is a war in which laws of war are disregarded. WWII can be considered a total war because of the level of national mobilization, the scale of of the armies, navies, and air forces raised through the active targeting of civilians and disregard for damage.