Four years later, just after Hitler came to power, Pius XI signed another treaty with Germany.

NPR: Vatican Opens Pre-WWII Papal Archive

Pacelli was the Vatican's envoy to Germany and later its secretary of state under Pius XI.

NPR: Vatican Opens Pre-WWII Papal Archive

The documents of Pius XI should also shed light on Eugenio Pacelli, who would become Pope Pius XII.

NPR: Vatican Opens Pre-WWII Papal Archive

The Vatican has opened the archives of Pope Pius XI, who presided over the Roman Catholic Church from 1922 to 1939.

NPR: Vatican Opens Pre-WWII Papal Archive

Pope Pius XI made the first papal radio broadcast in 1931.

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Church historian Alberto Melloni says the newly available documents should provide greater context and help explain the shifts in attitude of Pius XI regarding Europen politics leading up to WWII.

NPR: Vatican Opens Pre-WWII Papal Archive

In 1938, as he was growing even more concerned over what was happening in Germany and the rest of Europe, Pius XI asked an American Jesuit who was known for his anti-racist views to prepare a draft encyclical directly condemning racism and anti-racism and anti-Semitism.

NPR: Vatican Opens Pre-WWII Papal Archive