A new biography of Reinhard Heydrich argues that decisions to exterminate the Jews were not made in the 1930s but developed in stages, often in response to changing political and military circumstances.

At the state funeral following the assassination of Reinhard Heydrich in Prague in May 1942, Heinrich Himmler remembered his colleague as a noble man, “with a character of the rarest purity and a mind of penetrating logic and clarity,” rightly “feared by the sub-humans and slandered by Jews and other criminals.” Visibly moved, Adolf Hitler added that Heydrich had died as a martyr “for the preservation and protection of the Reich.” The Führer decorated him with “the highest award in my gift, the highest stage of the German Order,” and patted the cheeks of his two sons as he left the ceremony.

Chief of the Criminal Police, the SS Security Service, and the Gestapo, and ruler of Nazi-occupied Bohemia and Moravia, Heydrich was a key planner of the genocidal “Final Solution.” And yet, Robert Gewarth, a professor of history at University College Dublin, points out that no serious scholarly biography of him exists. Hitler’s Hangman fills this gap with a careful, compelling and chilling account of the Third Reich’s “administrator of death.”

Employing “a cold empathy,” Gewarth does not downplay Heydrich’s responsibility for his actions or the twisted “morality” he used to justify them. But he reaches beyond characterizations of Heydrich as a depraved monster or a “perversely rational desk-killer.” Dismissing as untenable assertions that Heydrich planned the Holocaust from the 1930s onward, Gewarth argues that decisions to exterminate the Jews developed in stages, often in response to changing political and military circumstances....