Richard Rhodes in Masters of Death: The SS-Einsatzgruppen and the Invention of the Holocaust (2002):

• The Great War set the stage for the Holocaust. The foul trenches (‘the long grave already dug,’ the English poet John Masefield called them), the no man’s lands of barbed wire, the piles of rotting corpses, the muddy, denuded landscape prefigured the killing pits and the death camps of the Third Reich.

Not merely "set the stage." The Holocaust re-enacted the First World War, with Jews replicating the pathetic fate of the soldiers of the First World War. Hitler taught us something about conventional warfare by creating an unconventional war against the Jews: massive, horrific death, but without righteous words like honor, devotion to one's nation, etc.

Richard Koenigsberg

PS: Although nine-million men died in the First World War while six-million human beings died in the Holocaust.