To what extent was World War II a war against civilians?
Sadly, it is quite easy to argue that World War II was a war against civilians. To some extent, this was due to the highly racialized nature of the war, which made it possible for some of the nations involved to inflict cruelties on civilian populations on a truly astonishing scale. The most visible example of this was the Holocaust, in which Nazi Germany attempted to exterminate European Jews. Over six million were either shot, gassed in death camps, or killed by starvation and disease as they worked as slave laborers. This was, it must be emphasized, the direct result of deliberate policy at the highest levels of the Nazi regime.
Another horrible example of war against civilians was the "rape of Nanking." Japanese soldiers raped, murdered, and tortured more than half of the population of Nanking, China from 1937-38. At least 300,000 people, mostly innocent civilians, perished at the hands of Japanese soldiers. While this event occurred before the formal outbreak of World War II, Chinese casualties in the resistance to the Japanese invasion are usually included among Allied totals. Another atrocity against civilians was committed by the Soviet Red Army when they invaded Nazi Germany. Angry at the horrors visited on their people by the Germans, they engaged in the widespread and systematic rape of thousands of German women.
Finally, there was the practice of strategic bombing carried out by the Allies in the latter years of the war. Anxious to bring the conflict to an end, American and British war planners began the bombing of civilian population centers in a way that brought about enormous civilian casualties. Especially horrific was the use of incendiary bombs against Dresden in Germany and Tokyo (and many other cities) in Japan. These terrible weapons caused firestorms that swept through the cities, incinerating tens of thousands of civilians. The policy of strategic bombing culminated with the destruction of Hiroshima and Nagasaki (and tens of thousands of innocent people) by atomic bombs in 1945.
Please be clear that this answer is not intended to suggest an equivalence between the actions of Nazi Germany, which were aimed at the total extermination of a people, and those of the Allies, which were aimed at bringing about the end of a terrible war. But the fact is that all sides in World War II waged war against civilians. It was a truly total war that in the end claimed the lives of possibly up to 100 million civilians, far more than combatants. So in large part, World War II was a war against civilians.