Janssen had made him aware of
Leopold von Ranke's
History of the Popes. This determined the field he would take for his own, becoming in a sense a Catholic anti-Ranke. His approach was that the apparent shortcomings of the Papacy reflected flaws of their times. Pastor consulted archives throughout Catholic Europe and, during his first trip to Italy in 1881, his seriousness ensured the patronage of
Pope Leo XIII, who opened to him the contents of the
Vatican Library, which had previously been held unavailable to scholars. He was granted privileged access to the
Secret Vatican Archives, and his history, largely based on hitherto unavailable original documents, superseded all previous histories of the popes in the period he covered, which runs from the
Avignon Papacy of 1305 to
Napoleon's entrance in Rome, 1799. He also investigated the archives of public and private libraries and archives in Italy and Europe, demonstrating an extraordinary capacity for work and rare sagacity in discovering interesting documents. Pastor decided to begin his work with the papacy of
Pope Clement V (1305–1314) and the onset of the
Avignon Papacy, so that he could concentrate his research on surviving documents. His dispassionate and frank papal history concentrated on individual popes rather than on the developments of papal institutions. Pastor's tomes span the pontificates of 56 popes, from Clement V to
Pius VI. He combined painstaking scholarship with erudition. The result of his research was his
Geschichte der Päpste seit dem Ausgang des Mittelalters in sixteen volumes. The
opus magnum was subsequently translated into English and published as
History of the Popes From the Close of the Middle Ages. Pastor began his work in 1886 and wrote throughout the pontificates of
Leo XIII,
Pius X,
Benedict XV and
Pius XI, publishing fifteen volumes. The 16th and final volume was published posthumously in 1930. The English translations were published between 1899 and 1953. ==Academic memberships, honours and offices==