The papacy made millions of diplomatic letters, private correspondence and other letters available on Monday to researchers interested in the 1922-1939 papacy of Pope Pius XI.
The Vatican has long sought to defend Pius' successor - the wartime Pope Pius XII, who had also served as a Vatican diplomat in Germany and later as Pius XI's secretary of state - against claims he did not do enough to save Jews from the Holocaust.
Researchers said it could take months or years to study the contents of 30,000 bundles of documentation from a period when fascism, nazism, communism and nationalism gripped Europe.
Archives officials said that by late morning around 50 researchers had shown their credentials to gain admittance.
"There was a bit of chaos," said Alessandro Visani, a researcher in contemporary history at Rome's La Sapienza university, who, like many others, was hoping for an initial idea of what was in the files.
One question revolves around an encyclical (a letter from the pope to all Roman Catholic bishops) Pius XI commissioned to denounce racism and the violent nationalism of Germany.
The encyclical was never published "in part because of his death and in part because it was judged to be inopportune politically," Visani said.
However, it appears that some archive items are missing. "I was stunned," said Emma Fattorini of La Sapienza.
"We can't find various versions" of drafts researchers expected to find, she said.
Lutz Klinkhammer, a German researcher in Rome, said he didn't expect any major discoveries concerning relations between the Vatican and Nazi Germany since the Vatican made available three years ago documents from the offices of the papal nuncios in Berlin and Munich during the Pius XI papacy.
The nuncio archives were made available ahead of schedule in a bid to deflect contentions from Jewish groups and others that the Vatican was silent in the face of the Holocaust.
"The facts are known, more or less. You look for details and shading"
Alessandro Visani, researcher
The Vatican maintains Pius XII used discreet diplomacy that saved thousands of Jews.
"We are not expecting any document to give us a scoop," said Klinkhammer.
The documents are expected to add subtle nuances to the research. "The facts are known, more or less. You look for details and shading," said Visani.
Nonetheless, the Giovanni Sale, a priest and historian at the Jesuit magazine Civilta Cattolica, is confident the archives will yield evidence to "correct" suspicions of anti-Semitism surrounding Pius XII.
The archives will provide "a new beginning for a history without prejudice," Sale told AP Television News.
Klinkhammer, who specializes in World War II, nazism and fascism, said he expects the files will enlighten scholars on relations between the Holy See and Europe's authoritarian states in the 1920s and 1930s, including Italy and Spain.
However, he added that a complete study of the Church's attitude toward the racial laws would require the opening of files from all of 1939, at least, when the matter was in "full discussion" among the clergy's ranks.
The files released on Monday end in February 1939, the month Pius XI died.
Ramon Corts i Blay, a priest and researcher at the Spanish National Church in Rome, described the newly opened files as "absolutely important" to his work in understanding church dealings in Spain, from its military dictatorship in the early 1920s to the Spanish Civil War.
Until Monday "there was a great gap" in research material for the period, said Corts i Blay.
Vatican archivists started to put in order the material spanning Pius XI's papacy in 1985.