27,000 French people were killed on 22nd August 1914, the bloodiest day in French history.This is four times more than at Waterloo, and as many in total as during the eight years of the Algerian War. Even more than the Battle of the Marne, Verdun or the Chemin des Dames. How did these men perish? In what circumstances? Does this deadly cataclysm at the very beginning of the conflict reflect the consequences of poor individual and collective choices, tactical, strategic or organizational mistakes, or quite simply bad luck?A record number of deaths in a single day unprecedented in French history cannot be a mere statistical oddity. It is the ambition of this work to provide some explanations, as well as ideas for how military strategists of the twenty-first century can avoid the combat lethality of the previous century.