The entire era of "the Hitlerian Wars", beginning with the Sino-Japanese War, the Italian invasion of Ethiopia and the Spanish Civil War was an era in which warfare was directed at civilian populations as a matter of official policy. The full outbreak of what we call World War II led to the deaths of millions of civilians who were attacked simply to break the national will. This was followed by both the Japanese and Germans applying terror, imprisonment and horrifying torutres and conditions to civil populations, largely driven by racist ideals. The Nazis ran infamous concentration camps, in which unknown millions were imprisoned and approximately eleven million were killed. About five million were Jews from all across Europe, but other minorities such as Gypsies and religious minorities such as Jehovah's Witnesses were also victims. The largest group were ethnic groups considered "subhuman" by Nazi ideology, and political opponents of the Nazis ( and their families). The single largest nationality in the camps were Germans whose own government interred them.
The horrific conditions in the concentration camps and the use of the prisoners as slave labor terrified the world. German corporations and businesses owned by the SS used not only the labor of prisoners, but their bodies as products, such as fertilizer and the production of consumer products using fats and hair. In some of the camps the mass murders were necessary to keep production rates up high enough, but uncounted thousands were murdered for no reason other than their ethnicity or political beliefs.
The Japanese bombed and shot and bayonetted civilians as a matter of course through the war all across China, the Philipines and Southeast Asia. A horrified world responded with total war against these countries and their allies, and in the aftermath of the war the UN framed the Universal Declaration of Human Rights.