Content Warning: This Index of Terms section references acts of genocide, racial violence, and murder that were perpetrated under the Nazi regime and are discussed in Into That Darkness.

The Vatican released a collection of papal documents from World War II, titled Actes et Documents du Saint Siège relatif à la Seconde Guerre Mondiale (Acts and Documents of the Holy See Relating to World War II) in 1965 in an effort to counter the developing narrative that Pope Pius XII remained silent despite knowing about the Holocaust. The collection isn’t exhaustive: as Sereny learned, it omits the most damning correspondences between the Polish ambassador to the Vatican, Kazimierz Papée, and Pius XII. In these letters, Papée detailed the Nazi’s plan of extermination and informed the Pope of the millions already killed. Despite this information, Pius XII remained silent on the ongoing genocide.

Aktion Reinhardt was the Nazi operation to exterminate Polish Jews, named after Reinhard Heydrich, one of the Holocaust’s architects. This operation, which spanned from October 1941 to November 1943, marked the deadliest phase of the Holocaust, during which the Nazis murdered an estimated 2 million Jews. This saw the establishment of four camps in Poland used exclusively for murder: Chelmno, Sobibor, Belzec, and Treblinka.