Buried in the files of the Secret Archives of the Vatican, there is a report from 1853. It was written by the first papal envoy to the United States, Bishop Gaetano Bedini, who recounted a telling episode. He was celebrating Mass in the chapel of the Presentation Convent, in the Georgetown neighborhood of Washington. A Protestant woman entered the church. Asked why she was there, she candidly answered that she wanted to check if it were true that Pope Pius IX's officials had horns on their heads.

On Tuesday, a century and a half later, a pontiff will enter the White House as a head of state and revered guest of the president of the United States. Benedict XVI's arrival marks a historic event: the first official visit since the establishment of full diplomatic relations between the United States and the Vatican just 24 years ago. Only John Paul II had been at the White House before -- on Oct. 6, 1979 -- but his meeting with President Carter was an unofficial one.

It took centuries to get to this point. The story of relations between the papacy and the U.S. is one punctuated by diplomatic and religious disputes and of frustrated attempts to establish formal ties. From the beginning, the Vatican viewed the New World as a "land of mission." It wanted to spread the Catholic faith among the fast-growing population and to establish ties with a country that clearly was set to become a world power.

But that wasn't easy. Catholic priests in the early United States preached in Latin or French because they spoke no English. As a consequence, they made few converts. Among Americans, Catholicism was perceived as a religion fine for Irish, Italian, French and Polish immigrants, but not for real Yankees. Or worse, it was seen as a religion of the poor. The Vatican also failed to grasp how widely the pope was viewed as a conspiratorial figure, menacing American freedom and independence. Year by year, the number of American Catholics grew, but the Vatican made few political inroads.

Dates can be annoying, but if one wants to pinpoint important mile-markers in the history of U.S.-Vatican relations, there are at least three. The first: 1867. At that time, Washington had just a "special legation," not an embassy, in papal Rome -- mostly for the purpose of keeping an open ear to the "intelligence emporium" that was the Holy See during this period of rapid social and political change in Europe. But tensions between the papacy and the community of American Protestants in Rome -- who were forced to move their church outside the city walls -- undermined bilateral relations. In February of that year, Congress cut off funds for the Rome legation, ending what de facto diplomatic relations existed. But there was a further, unacknowledged reason: The pontifical state was about to be conquered by the troops of the emerging nation of Italy. Washington considered the Vatican a failed state on the brink of extinction.

The second date is 1939. That year, President Franklin D. Roosevelt sent a personal representative to the Vatican. Officially, it was a "humanitarian mission," but in truth the U.S. wanted to get a close-up view of Fascist Italy, Hitler's ally in the Mediterranean. Another alliance, largely invisible, developed between Roosevelt and the new pope, Pius XII. Their connection had been carefully crafted three years earlier by an American cardinal, Francis Spellman, who arranged a secret meeting between the president and the future pope at the home of Roosevelt's mother in New York. That evolved into an anti-communist alliance between the White House and the Holy See that lasted throughout the Cold War.

Full diplomatic relations, however, were destined to be postponed. To the great disappointment of successive popes, subsequent presidents feared a furious Protestant backlash if they sent an ambassador to the Holy See. Even the only Catholic president, John F. Kennedy, kept the Vatican at arm's length.

Beyond religious and ideological obstacles, there was also a deep misunderstanding of the nature of the Holy See. The U.S. viewed the papacy as a big player in Italian politics but not on the global stage. Yet the Vatican was a soft-power player par excellence. By the 1980s, the Vatican had 102 accredited ambassadors (today 176), but not one to the United States.

Year after year, these two parallel empires grew -- both achieving global reach -- and overlapped on many international issues. But they did not recognize each other.

Until that third date, 1984. That's when President Reagan agreed to send an ambassador to the Vatican and to accept a nuncio, the Holy See's equivalent. It was a reward of sorts for the Vatican's strong and subtle support in the battle against the "evil empire" of the Soviet Union. And the Holy See was satisfied that Reagan and Congress had finally banished what was perceived as the remnants of religious prejudice against the pope.

When Benedict XVI and President Bush meet this week, surely neither will have forgotten John Paul II's 2003 rebuke of the Iraq war. But neither man has motive or desire to kick that dust up again. They have plenty of areas of common concern. Islamic fundamentalism is a lethal threat to both, although they often differ on how to confront it. The Vatican is expected to play a key role in any Cuban transition out of communism. Most pressing is the push to restore a whiff of normality in Iraq: Bush for geopolitical reasons, the pontiff for geo-religious ones, namely the endangered presence of Christian minorities in that region.

Today, the Vatican's "intelligence emporium" also has new value -- strategic connections in the Muslim world. Its global network of priests, nuns and missionaries gives the Catholic church eyes and ears in areas where the U.S. is not present, not trusted, not liked.

Benedict XVI's White House visit at last signals the true normalization of U.S.-Vatican relations and arrives at a difficult time for both parties. In a multipolar world, they can no longer exercise hegemony, whether political or religious. In fact, they need each other.

Massimo Franco is a political columnist for the Italian daily newspaper Corriere della Sera. He is also author of the forthcoming book, "Parallel Empires: The Vatican and the United States, Two Centuries of Alliance and Conflict."