In February 1960, at the height of the Cold War and in the decade of its deepest rebellion against Christianity, Nikita Khrushchev, the Soviet leader, approved a top-secret plan to destroy the moral authority of the Vatican in Western Europe and counteract the strong anti-communism of the Catholic Church. We know this first-hand from Ion Mihai Pacepa, one of the main leaders of Romanian intelligence and the highest-ranking communist soldier to defect to the US. He went into asylum in 1978 and recounted in detail the communist disinformation operations during the Cold War in his book "Disinformation."
Until 1960, the KGB had fought against the Church in Eastern Europe, claiming that it was a cesspool of espionage in the pay of US imperialism. But now Moscow wanted the Vatican leadership itself to be discredited, foisting on it the larger accusation of being a stronghold of Nazism. And Eugenio Pacelli, who was Pope Pius XII, was selected as the target of the attack. Pacelli was the ideal target because he had been papal nuncio to Germany, Pope during World War II, and had died in 1958, leaving him unable to defend himself.
The KGB's strategy was to present him as an anti-Semite and Nazi sympathizer who had encouraged Hitler's Holocaust. However, all the dirty work had to be carried out by Western hands, creating evidence from documents from the Vatican itself. Other disinformation operations had failed because the forgery did not have original documents as its starting point. The KGB needed some original Vatican documents, even if they were only remotely connected to Pius XII, to allow experts to slightly modify them to prove Pius XII's collaborationism. The operation against Pius XII was called "Seat 12," and Pacepa became the key figure because he was a Romanian government official.
The narrative was devised that Romania was ready to restore its broken diplomatic relations with the Holy See. To effect this supposed change of direction, the Romanians requested access to the Vatican archives to find historical roots that would help the government publicly justify its change of heart towards the Holy See. In turn, the Romanians promised to grant an interest-free loan of one billion dollars for 25 years, thinking that "if there is something that monks understand, it is money." Pacepa, as a Romanian government official, had a meeting in Geneva with an influential member of the diplomatic corps, Agostino Casaroli, who would later become Vatican Secretary of State under John Paul II. As part of the deal, Casaroli gave him access to the Vatican archives. So Pacepa sent three young undercover intelligence officers posing as Romanian priests to rummage through the papal archives and take clandestine photos of the materials that could be used for the operation. This part of the plan was executed while the loan and the reestablishment of diplomatic relations fell through because it was all a hoax.
Between 1960 and 1962, Romanian intelligence managed to steal hundreds of documents somehow related to Pius XII from the Vatican Archives and send them to the KGB. But Pacepa says that incriminating material against the pontiff never appeared. However, in 1963, General Ivan Agayants, head of the KGB's disinformation department, developed the "Seat-12" operation by creating a book and a powerful play, The Vicar, which revealed Pius XII as a Nazi sympathizer.