Remaining in the army, he was sent to Bavaria to fight against left-wing revolutionaries. (And yet Mr. Weber has discovered that, briefly at the turn of 1918-19, and unmentioned in “Mein Kampf,” Hitler wore a red brassard and supported the short-lived Bavarian Soviet Republic.) Demobilized, he became an informer for the army’s propaganda unit— though whether he volunteered or was coerced because of his short-lived involvement with the Bavarian Soviet Republic…