In 1939, the pope ordered the death of Adolf Hitler.

Church of Spies by Mark Riebling tells the gripping history of the Vatican’s covert operations during World War II. A military plot had formed in Germany to depose the Führer, but there was a sticking point: what might happen to that country after Hitler was no more. The aftermath of World War I saw Germany punished severely, which helped spring Hitler to power in the first place. Nobody wanted a repeat of that, and so the conditions for mounting a coup were established: If the world promised a “just peace” for a de-Nazified Germany, the generals would go through with their plan and have Hitler killed.

The problem was a lack of assurances: Hitler’s enemies at home had no way of knowing if his enemies abroad would abide by the agreement. His enemies abroad, meanwhile, had no way of knowing if they weren’t about to replace one tyrant with another. The only person with the prestige and freedom to act was the pope, but asking the pontifex maximus for the go-ahead to put a bullet in someone’s brain was a tall order. Ultimately, not only did the pope say yes when approached, but he also established a robust intelligence apparatus and kept pushing parties to get on with it.

Pope Pius XII’s relative silence during the Holocaust was criticized as both a moral failing and an inexplicable change of public demeanor. Before being elected pope, Eugenio Pacelli, as he was born, was a fierce critic of national socialism, sharpening his predecessor’s encyclicals and preaching racial equality. As pope, only his first encyclical during the war mentioned the Jews by name, and for such a fiery enemy of the Reich, he seemed to fall relatively—and inexplicably—silent on the issue. In fact, after the publication of that encyclical, as Riebling explains, “The last day during the war when Pius publicly said the word ‘Jew’ is also, in fact, the first day history can document his choice to help kill Adolf Hitler.”

Pius XII resolved to do whatever he could to have Hitler killed. His fellow plotters in Germany’s intelligence and military services asked him to keep quiet: “Singling out the Nazis,” later said one conspirator against Hitler, “would have made the German Catholics even more suspected than they were and would have restricted their freedom of action in their work of resistance.” The Catholic Church was a powerful resource to be leveraged. Though the Vatican lacks a formal intelligence service, during the war it possessed a de facto one: priests, monks, and nuns embedded in even the most war-torn towns in Europe, and their ability to secretly filter information to Rome, which could then disperse it widely or to necessary parties. In other words: The Church was a kind of ecclesiastic telegraph. ...