Robert Leiber, S.J. was a close advisor to Pope Pius XII, a Jesuit priest from Germany, and Professor for Church History at the Gregorian University in Rome from 1930 to 1960. Leiber was, according to Pius's biographer Susan Zuccotti, "throughout his entire papacy his private secretary and closest advisor".

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.