Vatican City ? The Vatican opened part of its secret archives Monday to let historians review millions of diplomatic letters, private correspondence and other church documents to gain insight into how the Holy See dealt with the growing persecution of Jews before World War II.

Researchers said it could take months or years to study the contents of some 30,000 bundles of documents from the 1922-39 papacy of Pius XI, a span when the rise of Nazism, Fascism and Soviet-bloc communism gripped Europe.

The opening is part of the Vatican’s efforts to defend Pius’ successor, the wartime Pope Pius XII, against claims he did not do enough to save Jews from the Holocaust during the wars.

The Vatican insists Pius XII, who earlier served as a church diplomat in Germany and later Vatican secretary of state under Pius XI, used discreet diplomacy that saved thousands of Jews.