Pope Pius XII was head of the Catholic Church and sovereign of the Vatican City State from 2 March 1939 until his death in October 1958. Before his election to the papacy, he served as secretary of the Department of Extraordinary Ecclesiastical Affairs, papal nuncio to Germany, and Cardinal Secretary of State, in which capacity he worked to conclude treaties with various European and Latin American nations, including the Reichskonkordat treaty with the German Reich.

During the Second World War, Pope Pius XII maintained links to the German resistance to Nazism against Adolf Hitler's Nazi regime. Although remaining publicly neutral, Pius advised the British in 1940 of the readiness of certain German generals to overthrow Hitler if they could be assured of an honourable peace, offered assistance to the German resistance in the event of a coup, and warned the Allies of the planned German invasion of the Low Countries in 1940. The Nazis considered that the Pope had engaged in acts equivalent to espionage.