I have written a number of articles that mention how Adolf Hitler's differences with the Pope were similar in nature to prior differences between Holy Roman Emperors [who were almost always German] and Popes. [See my Adolf Hitler page for a list of articles]
Here I want to go into more detail, expanding the thesis that Adolf Hitler was Holy Roman Emperor 2.0. This is not a trivial exercise. Hitler's armies came perilously close to winning World War II. If they had, there would only be one official religion today: Roman Catholicism. It would likely be a Nazi-modified, Catholicism. The extent of modification would depend on how the power struggle between the Pope (who had engineered Hitler's rise to power) worked out. Then, there would have been the power-struggles between their successors.
Nor are we today free of the prospect of another attempt to revive the Holy Roman Empire. There are also parallels with other religious zealots, such as militant Islam and, in the United States, militant Protestantism.
I also think that it is important to closely associate Hitler with Roman Catholicism in the public mind. I have yet to meet a Roman Catholic in the United States who even knew, before I told them, that Hitler was a Roman Catholic. And they pretty much go into denial after they are informed of the fact.
As in the analysis of all things complex, historians differ somewhat on where they place the beginnings of the Holy Roman Empire. All distinguish the Holy Roman Empire from the older Roman Empire, although there was a chain of continuity between them. The old Roman Empire had become Christian as it fell into decline after the capital was moved from Rome to Constantinople. The Bishop of Rome was, in the era between about 200 A.D. and 600 A.D., prestigious but not widely considered to be the head of the Church. While various legends were concocted to increase the power of the Roman bishop (including the idea that the Apostle Peter started a chain of succession of universal church dictators in Rome) over the centuries, what really resulted in the assumption of power was the destruction of the more-ancient African and Middle Eastern churches during the Islamic conquests of the 7th century.
[for details of the evolution from the old Roman Empire to the Holy Roman Empire see The Holy Roman Empire by James Brice]
The creation of the Holy Roman Empire was marked by the crowning of its first Emperor, Charles the Great a.k.a. Charlemagne, by Pope Leo III. There are two points about this that are missed by modern commentators. First, Charlemagne, leader of the Franks, was a German, and a very successful militarist German at that. Second, Leo III was the first Bishop of Rome to be able to assert, with Charles's army at his back, that he was the supreme leader of all Christians, thus making the term catholic factual. Needless to say the bishops of the more-ancient Orthodox churches like Constantinople and Antioch did not accept this assertion. They had their own, better-documented, stories of apostolic succession.
The Holy Roman Empire is also called the First Reich. It is generally accepted to have ended in 1806. It was always Roman Catholic. Its history was punctuated by often bitter disputes between the Emperor and the Pope, sometimes involving clashing armies.
The Second Reich, or German Empire, was not officially Catholic, was more clearly German, and lasted just a few decades, from 1871 to 1918. There was less pretense to the right to global domination, although the Germans had defeated France in 1870 and did build up a set of colonies, though they were dwarfed by the British Empire.
Adolf Hitler was born near the German border in Austria in 1889, in a Roman Catholic family. After trying his hand as an artist, he volunteered to join the German army during World War I, and became a German citizen.
Hitler's focus was on restoring Germany to power through the National Socialist, or Nazi, party. The two largest religions in Germany at the time were the Lutheran Church and the Roman Catholic Church, with northern Germany tending to the Lutheran and southern Germany tending to the Catholic. Conveniently described after his defeat as an atheist, in fact Hitler was the champion of most people opposed to atheist Socialism and Communism.
Although the Nazis became the largest electoral party in Germany, the one thing the other political parties could agree on, until 1933, was that the Nazis should be excluded from power. But Pope Pius XI, along with the future Pope Pius XII (who was then the papal representative in Germany), thought otherwise. They needed a new Holy Roman Emperor to fight modern ideas like atheism, socialism, and even Protestantism. Mussolini did not seem to be up to the task. They cut a deal, through the Catholic electoral party in Germany and one of its leaders, Franz von Papen, who was a Papal Chamberlain and had been Chancellor of Germany in 1932. Under the agreement Hitler became Chancellor with Papen as his Vice-Chancellor. As part of the deal Hitler quickly agreed to Papal demands for a Reichskonkordat.
Hence the Third Reich. But Charlemagne had not been made Holy Roman Emperor just for inheriting the Frankish throne and being a good Christian (if, like most Christians, you think war and your religion are compatible). No, Charlemagne defeated a bunch of weaker tribes and kingdoms, and then defeated a specific enemy of Leo III, the Lombards, before he was worthy of the imperial crown.
Hitler had some work to do first. He got Germany's economy back on track (this was during the Great Depression), the real reason Germans were so fond of him. He built up an army and air force. Then he used diplomacy and the threat of force to get back areas taken from Germany at the end of World War I.
Meanwhile, General Franco seized Spain in a bloody civil war, leaving France as the only basically Roman Catholic, but non-fascist, nation in Europe. A generation of Popes had schemed to re-Catholicize Europe, and the dream seemed to be coming true.
Later, Catholic propagandists would point to arguments between Hitler and some Catholics, including Pius XI and Pius XII. But these were arguments within the budding new Holy Roman Empire; not really different (and in some cases remarkably identical) to arguments between earlier Emperors and Popes. Either pope could have tried to order Hitler around by threatening excommunication. But neither did. Hitler remained officially Catholic until the day he died. The Pope certainly did not object when Hitler forced the Lutheran and Roman Catholic churches in Germany to unite.
Hitler frequently said things critical of the Church or specifically of some of her clergy. Again, later apologists would say that he was not really Catholic, whatever his official status. But go back to Charlemagne and you will find the same details. In fact, Charlemagne bossed around the Catholic clergy, and even the Pope, far more than Hitler ever did. And yet no historian today would say Charlemagne was not Catholic. In fact, he was made a Catholic saint.
The reason Hitler is not a Catholic Saint today is because the communist-atheists defeated him, with surprisingly little help from the British Empire. The United States only fought enough in Europe to grab what it could towards the end of the war.
The ideal of the Roman Empire was the rule of law under one man, the Emperor. The idea of a Universal religion was added to create the Holy Roman Empire. What Hitler and Pius worked out was a plan to globalize the church-state combo.
If you don't know enough about Adolf Hitler to judge the validity of my argument here, I would encourage you to start with The Rise and Fall of the Third Reich by William Shirer. Then read about the close relation of the fascist movement to the Roman Catholic Church.