I thought 'harming civilians' is the fairly recent development. Battles usually always used to be between two armies on a field, away from cities. At the beginning of WWII, bombing cities was not even considered by the Allies; by the end, they were doing it themselves, justified by the Germans V2s bombing cities in Britain. The Germans justified that by saying the WWI British blockade of Germany was aimed at starving the people and killed many more. In the Nuremberg trials, the standard was "stuff they did that we didn't", but that standard had been slipping constantly during the century, as that book makes plain.
Perhaps most frightening was the description of the 'decision' to drop the atomic bombs on Japan, which wasn't even a decision, but a series of misunderstandings between a few different groups. No-one made a decision.