While Cornwell [author of Hitler’s Pope] was granted access only to the Vatican’s open archives, he claimed that his book was based on “previously unseen material.” He also said he spent “months on end” in a “windowless dungeon beneath the Borgia Tower,” while a “silent factotum brought him Pacelli’s files, which had been hidden from view for decades.”
In fact, the files were simply in an underground storage vault; moreover, they were not secret, and they covered the years 1912-1922, before Hitler was running Germany and while Pacelli was nuncio to the Kingdom of Bavaria. Vatican records show that Cornwell visited those archives only from May 12 to June 2, 1997, that he did not come every day, and that he often stayed for very brief periods of time…He later admitted that he was there only three weeks and that the files were not secret – but by then the damage was done.
In his preface, Cornwell claimed that by mid-1997, “nearing the end of my research, I found myself in a state I can only describe as moral shock” over the evidence that Pacelli had an “undeniable antipathy toward the Jews” and that his “diplomacy in Germany in the 1930s” betrayed groups that “might have challenged Hitler’s regime and thwarted the Final Solution.” […]
The most revealing insight into Cornwell’s research methods lies in the fact that, citing a secondary source, he wrote that the papal nuncio in Vienna had warned Pacelli about the risks posed by the concordat. Remarkable, Cornwell made no mention of the original document from the Vienna nuncio, which Cornwell is shown to have signed out while doing research in the Vatican archives, and which directly contradicts the statement in his book.
Lt. Gen. Ion Mihai Pacepa and Professor Ronald Rychlak, Disinformation: Former Spy Chief Reveals Secret Strategies for Undermining Freedom, Attacking Religion, and Promoting Terrorism (2013)