In every war, some soldiers obey the rules and others break them. And in every war, there are differences between the rules of war and what armies order, encourage, or allow their soldiers to do. The 1907 Hague Convention and 1929 Geneva Convention set the rules applicable in World War II: Civilians were to be left alone as much as possible, and when an enemy soldier surrendered he was to be removed from battle, given medical attention, and adequately housed and fed. All the countries that participated in World War II violated the rules to some degree, but for the Nazis, killing POWs and civilians was central to their military objectives —to give Germany Lebensraum (living space) and safe borders, ending any threat from the "subhuman" Jews and Slavs. Thus, German soldiers were ordered to kill civilians and POWs, sometimes through explicit instructions, at other times through...