When Pope John Paul II first announced the opening of Pius XI's archives, what made you think there might be an untold story buried inside?

The Vatican’s alliance with Mussolini has long been controversial. Historians and journalists formed two camps. On one side were those who claimed that, far from being an ally, the Vatican was Mussolini’s greatest adversary during the twenty years of the Fascist regime. On the other side, people charged that the Church offered the regime crucial support. Yet until the 2006 opening of the Vatican’s archives—and with it a series of other Church archives—the controversy remained unsettled.

Mussolini’s infamous Italian “racial laws” of 1938 became the main flash point of these debates. The laws branded Italy’s Jews a dangerous, foreign race, and introduced a series of draconian anti-Semitic measures. Here, too, two very different accounts have been offered. One describes a pope who fought the racial laws with all his power, triggering widespread popular opposition to Mussolini. Proponents of the other argue that, far from being opposed by the Vatican, the anti-Semitic campaign relied heavily on Church support.

With the announced opening of the Vatican archives I thought the full story would finally come out. Having worked in the Vatican archives for earlier periods I knew how incredibly rich its documentation is. The Vatican takes its history very seriously and its records are massive.

What did you know about Pius XI before you began your research and how did that evolve?

I had written a book about the Vatican’s role in the rise of modern anti-Semitism that came out a dozen years ago. Originally I planned to have that book end with the death of Pius XI’s predecessor in 1922, because at the time the Vatican post-1922 archives were not open. But my editor convinced me that, given the great interest in the subject of the Church and anti-Semitism in the years leading up to the Holocaust, I should extend the book to cover that period. I was a bit reluctant, thinking that without the Church archives much of the story was unknowable.

Although Pius XI’s archives were closed, a lot could be learned about him from the records in the years before he became pope. Most telling was the rich documentation for the immediate postwar years, when he served as the pope’s envoy to Poland. There he both developed a visceral anti-Communism that would stay with him as pope, and he experienced the deep anti-Semitism of the Polish Catholic clergy.

It is up to the pope to decide when to open the next unopened papal archives to scholars. There is no 50-year rule, or 75-year rule, as is found in other sorts of archives. When John Paul II in 2001 authorized the opening of Pius XI’s archives (1922-1939) it was unclear how long it would take Vatican archivists to put the huge mass of documents in order and allow scholars in. But of course if you are interested in the relationship between the Italian Fascist regime and the Vatican, the relevant archives are not just to be found in the Church. There are also the Fascist archives and other Italian government documentation, not to mention diplomatic archives in France, Britain, the U.S., and elsewhere. These were already open and I began working in them. By the time the Vatican archives opened in 2006 I had already identified and digitized 15,000 pages of documents from these non-Church archives. Many documents told of encounters with Pope Pius XI. Others, based on conversations with the pope’s confidantes, similarly provided valuable insight into what Pius XI was thinking and doing, and to his personality. As a result, when I first began reading the newly available documents from the Vatican I already had learned a great deal about the pope.

Before coming to power, Mussolini's anti-church views were well-known. He frequently denounced church authority and ordered his thugs to beat up local priests. How did he end up embracing the pope and why would the Vatican trust such a man?

Until shortly before he came to power, in 1922, Mussolini was a strident anti-cleric. The first piece he ever wrote was titled “God does not exist.” He referred to priests as black microbes and called for seizing the Church’s property. When the Fascist movement was founded in 1919, it reflected this antipathy to the Church. But very soon after this, Mussolini, who was nothing if not an opportunist, realized that his path to power would be vastly aided by winning the Vatican’s support. Not only was 99% of Italy Catholic, but other than the Socialist Party, the main party standing in the Fascists’ way was the Catholic Popular Party, headed by a priest.

So the question is not so much why Mussolini might want the pope’s support, but why, as you ask, would the pope trust him. In fact, Pius XI never fully trusted Mussolini, and over the next years the pope, who had a volcanic temper, would periodically lash out at him in private.

But to understand why the pope ended up embracing Mussolini you need to understand how embattled the popes of these years felt. Modern Italy was created not that many decades earlier, capped by the seizing of Rome—capital of the Papal States-- in 1870. Since that time no pope had set foot outside the tiny confines of the Vatican. They branded the new Italian state illegitimate and denounced the new doctrine of separation of church and state. Mussolini promised to restore the Church and the clergy to a privileged position in Italy. The pope dreamed of making Italy into what he called a “confessional” Catholic state and planned to use Mussolini to help him accomplish his goal. The pope had no faith in democracy. In fact, his greatest fear was that the socialists and communists would take advantage of the multi-party chaos to come to power in Italy. He saw Mussolini as the best bet for preventing it.

The Pope and Mussolini is based on more than seven years of archival research. Tell me about one or two documents you uncovered that were breakthroughs in your understanding of these two men and this era.

There is nothing quite like holding in your hands a letter written by a major figure in the past, or his or her notes, to make you feel transported back in time to a scene of historical drama. While a lot of archival research is tedious, as you know, poring through masses of materials that turn out to be of little interest, all is more than made up by those times when you hold such a document in your hands, especially one that casts new light on a little known or poorly understood part of the past.

There were so many revealing documents, of so many different kinds, that it is hard to identify just one or two. Perhaps the most dramatic—what could even be called a kind of “smoking gun”—was the three-page text of a secret agreement between the Vatican and Mussolini reached two weeks before the racial laws were first announced. The trail of documents I unearthed shows the pope’s shadowy, but fascinating, Jesuit personal envoy to Mussolini, Pietro Tacchi Venturi, spending the days before the agreement going back and forth between the pope and the dictator to work out an accord. Shockingly, it states the Vatican’s agreement to make no objection to the racial laws as long as they were no more repressive than the popes’ own restrictions on the Jews in the days of the Papal States. And in fact the laws that were soon announced—expelling all Jewish students from the schools, firing all Jewish teachers, forbidding Jews from holding other positions of influence—were similar to those that had been in effect in Rome as long as the popes held power there.

But not all of the most revealing documents were to be found in the Vatican archives. We know more about what was going on behind the scenes in the Vatican in these years than for any other time in history thanks to the dense network of spies the Fascists placed in and around the Vatican. These too shed much light on the pope and what he was dealing with.

Let me give one example. No man was closer to the pope than his longtime master of ceremonies, who literally stood at his side every day. Yet a constant stream of secret police reports—found in Italy’s central state archives—document the monsignor’s penchant for luring boys to his Vatican apartment for sex. As it turns out, the man who stood on the pope’s other side during public events, the prefect of the papal household, was likewise accused of pederasty, and the pope unceremoniously dismissed him. He was not heard from again. But the master of ceremonies was a man the pope had known many years earlier in Milan and, given their close ties, he was dealt with very differently. By the time of the pope’s death, having become cardinal deacon, it was this man who would crown the pope’s successor, Pius XII. Reading the reports in the police archives of how, even after he had become cardinal deacon, he lured boys to his rooms and how the cover-up was organized opened up a whole realm of what was going on around the pope that no scholar, as far as I know, has ever uncovered. There is nothing more exciting for a historian than this.

What role did Pius XI's Secretary of State, Eugenio Pacelli, play in this relationship? Do you think that Pius XI had any idea that Pacelli would succeed him as Pius XII, the war- time pope who would become one of the most controversial in church history?

In 1929, Pius XI brought Eugenio Pacelli back to Rome—he had spent a dozen years as papal envoy in Germany—and made him a cardinal. Early the next year he appointed him his secretary of state, the number two position in the Vatican. In many ways Pacelli offered a stark contrast to the pope. Pius XI came from a modest northern Italian family, Pacelli from a highly influential Roman family close to the Vatican. Pius XI had been a mountaineer, and climbed Italy’s highest Alpine peaks. Pacelli was a bit of an ascetic beanpole. Pius XI was quick to anger, and the reports of the ambassadors who met with him are full of accounts of his banging his fist on his table and shouting at them. Pacelli always kept his cool. In fact, it became Pacelli’s role to try to calm the pope down when the pope thought someone or something was threatening the Church.

The story I tell in my book is of Pius XI’s increasing irritation with Mussolini, especially as he began to ally more closely with Hitler, whom Pius XI hated. Pacelli, along with his main collaborators in the Vatican, struggled to keep the pope from jeopardizing the Church’s alliance with the dictator. And so the book offers episode after episode of Pacelli urging the pope not to speak out publicly against either Mussolini or Hitler. And when the pope did speak out, Pacelli did what he could to keep his remarks from being published in the Vatican newspaper. Among the most revealing documents are those in which the Italian and German ambassadors to the Vatican record Pacelli’s expressions of despair when, increasingly, the pope paid so little attention to his advice.

All that said, the pope had long had a high opinion of Pacelli’s intelligence, his diplomatic skills, and his devotion to the Church. He also at some level realized the value of having someone able to prevent his temper from creating diplomatic problems. There is good evidence that he saw Pacelli as his most qualified successor. Yet in the last weeks of the pope’s life, there were various initiatives—involving criticism of Nazism and of Mussolini’s increasing imitation of Hitler—that he kept secret from Pacelli. He may have had an inkling of what Pacelli would do once he got his hands on the materials he was preparing.

In the final months of his life Pius XI began to realize he had made a poisonous bargain with Mussolini and fascism. He tried to change the course of the church's relationship to Mussolini and Hitler, but it proved too late and he died in February, 1939 as the world was sliding into catastrophe. How much do you think Pius XI understood about what was coming to Italy, Europe, and the church?

Pius XI was in many ways a tragic figure. His mentality was formed in a certain conservative Church ambience of the late nineteenth century. This viewed the modern world as corrupt and looked backward to the Middle Ages for its models of good Christian society. People should not act according to their own beliefs and conscience, but according to the directives of the Church hierarchy. Proper authority should not be questioned.

It was only after he had been pope for over a decade that Hitler’s rise to power in Germany and Mussolini’s own increasing efforts to portray himself as a demi-god began to challenge the pope’s worldview. Something similar might be said about his attitude toward the Jews. He came from a Catholic environment in which the Jews were not only demonized as the crucifiers as Christ, cursed by God, but viewed as part of an occult conspiracy aimed at enslaving Christians and achieving world domination. Yet in his own city of Milan, he had gotten along with the small Jewish community and indeed even took Hebrew lessons from the local rabbi. Watching how his views of Jews percolated in the years leading to the Holocaust is to see a man struggling with a conflict he does not entirely comprehend.

As for his understanding of what was coming by the late 1930s, the newly available archives make clear he was convinced that Europe was hurtling toward a cataclysm.

Do you think there was a moment where a road or course not taken could have changed things significantly?

A huge amount of attention has been paid to the question of the “silence” of Pius XI’s successor, Pius XII, during the Holocaust. This has turned into a rather heated debate over whether Pius could have affected German behavior by forcefully denouncing the mass murder of Europe’s Jews. I don’t want to get involved in that debate here, but what is clear to me is that the popes had much greater influence over Italians than they did over the Germans. Of course the popes themselves were all Italians, as were virtually all the members of the Curia. And while only a third of Germans were Catholic, Italians were overwhelmingly Catholic. So the interesting question for me is could the pope have prevented Italy from allying with Nazi Germany. Might Italy never have entered the war on Germany’s side if the Vatican had acted differently? This is a huge question and I am not sure if it has ever been posed in quite this way before.

Although I know it will be controversial, I think the Vatican might well have been able to prevent Italy from joining Nazi Germany in its genocidal war. Had it done so, the results for Italy would have been huge, for participation in the war as Hitler’s handmaiden devastated the country, both physically and morally, with effects that are still being felt.

Italians had no fondness for Germany or Germans. Only a couple of decades earlier German and Austrian forces had killed half a million Italians in the First World War. Mussolini himself had announced that he would never let Germany acquire Austria for, in the aftermath of the Great War, Italy needed an independent Austria to protect its northern border. The Italian king thought Hitler a lunatic and a degenerate. Meanwhile, Italians had good reason to be uncomfortable with the Nazis’ claim to be members of a superior race. Hitler had referred dismissively to Italians as of partially “Negroid” origin. Convincing Italians, from Milan to Sicily, that they were all members of a pure Aryan race would be far from easy.

Mussolini himself was desperate to keep the pope from denouncing his alliance with Hitler, and in this he had the help of Cardinal Pacelli. Had Pius XI clearly and publicly branded Nazism as pagan and the Nazi Party as criminal, had he excommunicated Hitler and his cronies, and had either he or his successor, Pius XII, made it clear that no good Catholic could fight in the Nazi war, it would have made it very difficult for Mussolini to move ahead. Indeed, in conversations just before the war with Nazi emissaries in Rome, Mussolini made just this point.

Jon Meacham is the Pulitzer Prize-winning author of American Lion, Thomas Jefferson, Franklin and Winston, and American Gospel. The former editor of Newsweek, he is an Executive Editor and Executive Vice President of Random House.