By David Willey
BBC News, Rome
The Vatican has opened to scholars and historians part of its vast collection of archives.
Pope Pius XI oversaw the Vatican's relations with pre-war Germany
The section being opened covers the rise of Mussolini and Hitler and the run-up to the World War II.
It dates from the reign of Pope Pius XI, who was pope from 1922 until just before the outbreak of war in 1939.
Details of the Vatican's relations with Germany's Adolf Hitler and Italy's Benito Mussolini are expected to be of great interest to scholars.
The Vatican houses one of the world's most important historical archives, the so-called "secret archive" which contains some 2m documents dating back up to 1,000 years.
They are kept for safety mainly in underground fireproof bunkers and storerooms placed along some 50 linear miles (80.4km) of shelving.
Periodically the pope decides to open to scrutiny by scholars and professional historians a new treasure house of documents.
The latest release covers the years when no less than four notorious dictators came to power - Benito Mussolini in Italy, Joseph Stalin in Russia, Adolf Hitler in Germany and Francisco Franco in Spain.
Details of relations between the Vatican and these countries during this period will be of outstanding interest to scholars.
The future Pope Pius XII was the Vatican's nuncio or ambassador in Berlin in the 1930s and his correspondence with Rome will also be eagerly scrutinised.
Pope Pius XI refused to meet Hitler when he visited Rome in 1938 at Mussolini's invitation.
The Pope ordered the Vatican museums to be closed and shut himself up at his residence at Castel Gandolfo until Hitler had left town.