The wartime leader of the Roman Catholic Church who has been described as "Hitler's Pope" was in fact a secret opponent of the Nazis and secretly plotted to kill Hitler, a new book claims.

Mark Riebling in Church of Spies says Pope Pius XII ran a secret war against the Nazi regime and did not speak out against the crimes against Jews and others because he feared this would hamper his efforts to undermine national socialism.

"When he learned of the Holocaust, Pius played his cards close to his chest. He sent birthday cards to Hitler while secretly plotting to kill him," the book claims.

Riebling's book counters the charges against Pius XII made by author John Cornwell in Hitler's Pope. Cornwell argued that Pius did not do enough to fight Hitler and instead furthered his own agenda of increasing the power of the Papacy. Cornwell also said Pius was anti-Semitic.

Riebling, a historian, instead says that Pius XII worked with the German resistance and others to plot Hitler's death. The plot failed for several reasons, he reports, but it was in the hopes that it might succeed that Pius XII did not criticise Hitler and the Nazi regime more openly, even though he had been a vocal critic of National Socialism before the war.

Hitler bore a hatred for the Catholic Church, warning against "assassins whipped up by the black crows in confessionals."

Pope Pius used a Catholic source, Josef Müller, to dig out information on Hitler's movements and carry messages back and forth. The book says  Müller was able to provide information on Hitler and also help German bishops communicate secretly with the Vatican. Müller's code name in the ingtrigue was Herr X and Pius XII was The Chief.

David Brown reports on MentalFloss that the Pope ordered the death of Hitler in 1939 and the generals were prepared to go through with the plan but the problem was a lack of assurances. 

He writes: "It was the Vatican's agents and allies who were so successful in everything from finding and leaking Hitler's plans for German invasion of Belgium, to helping orchestrate multiple attempts on the tyrant's life. And, as Church of Spies explains in extraordinary and well-documented detail, it all happened because Pope Pius XII had no qualms with killing the evilest man in the world."