That nickname, "Rock," belonged to Ray Rocca. Rocca served as Angleton's deputy in Rome and for most of his later career. His career included responsibility for the Central Intelligence Agency's records concerning the 1963 assassination of President John F. Kennedy.
"So, here's a guy who had been in the Vatican; who had been charged with penetrating the Vatican; and who knew a thing or two about assassination probes. I thought: here's an interesting guy to get to know," Riebling said. Rocca did not violate his oath of secrecy, but his interviews with Riebling are among the book's sources.
According to Riebling, his book does not charge that the Pope "tried to kill Hitler." Rather, the Pope's actions were more subtle.
"Pius becomes a key cog in conspiracies to remove a ruler who is a kind of Antichrist, because good people ask for his help, and he searches his conscience, and he agrees to become an intermediary for the plotters – their foreign agent, as it were – and thereby he becomes an accessory to their plots."
The historian described these actions as "some of the most astonishing events in the history of the papacy."
Pius XII had connections with three plots against Hitler. The first, from October 1939 to May 1940, involved German military conspirators. From late 1941 to spring of 1943 a series of plots involving the German Jesuits ended when a bomb planted on Hitler's plane failed to explode.
The third plot again involved German Jesuits and also German military colonel Claus von Stauffenberg. Although the colonel successfully planted a bomb near the Nazi dictator, it failed to kill Hitler. The priests had to flee after the failed attempt. Those unable to escape were executed.
During his research, Riebling discovered that Pius XII secretly recorded the conversations held in his office. Transcripts of the Pope's talks with German cardinals in March 1939 show that he was deeply concerned that German Catholics would choose Hitler instead of the Church.
"The cardinals asked Pius to appease Hitler, so that German Catholics won't break away and form a state church, as happened in Tudor England," Riebling said.
"Pius heeded the German episcopate's advice. Instead of protesting openly, he would resist Hitler behind the scenes."
Pius XII's agents provided the Allies with useful intelligence about Hitler's war plans on three occasions, including Hitler's planned invasion of Russia. In all three cases, the Allies did not act on the information.
(Story continues below)
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For their part, the Nazis regarded Pius XII with suspicion since his election in 1939.
"He worked hard to allay those suspicions, to minimize persecutions of German Catholics. But the Nazis never dropped their guard," Riebling said.
At one point Hitler planned to invade the Vatican, kidnap the Pope and bring him to Germany. Leading Nazi Heinrich Himmler "wanted to have the Holy Father publicly executed to celebrate the opening of a new soccer stadium," Riebling said.
"Pius became aware of these plans, through his secret papal agents; and, in my view, that influenced the Holy Father's decision to become involved with the anti-Nazi resistance."
For Riebling, the assassination plots against Hitler were an admission of weakness, "because it's saying that we can't solve the problem by some other means."
"Knowing what I do about Pius XII, and having researched him for many years, I believe he wanted to be a saint. He wanted people in Germany to be saints," he added.