259th Pope of the Catholic Church. Pontificate 1922 - 1939. Signed the Lateran Treaty with Italy, establishing the Vatican City as a sovereign state. Pius XI is remembered as the pope who reigned between the two great wars of the 20th century, fighting the two ascendant ideologies of communism and fascism. The onetime librarian also reorganised the Vatican archives. Nevertheless, Pius XI was hardly a withdrawn and bookish figure. He was also a well known mountain climber with many peaks in the Alps named after him, he having been the first to scale them.