Here's a factoid I've borne in mind a lot these last few years: when the 20th century opened, civilian casualties accounted for about 10% of all casualties in conflict. By the century's close that ratio had reversed - about 90% of all casualties of conflict are civilian casualties.

I doubt this factoid is true if you do not (as they did at the time) constrain the definition of war to the conflicts between european powers. When dealing with people without machineguns (Congo, Tasmania and the list goes on) attempted genocide was common. WW2 saw these practices spread to europe, hence the horror.

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