
Originally Posted by
Alexios Komnenos
He did. Innotentius the Third, the Iron Pope, a man who ruled over all of Catholic Europe, with influence everywhere. Possibly the most powerful Pope af all history, and also one of the best politicians ever. This man declared the Fourth Crusade, which objective was the taking of Egypt.
Once in Venice, the crusader leader, Boniface de Montferrato, was convinced by Enrico Dandolo, Doge of Venice, and Alexios Angelos, exiled from the Byzantine Empire and relative to the current Basileus, Alexios III Angelos, to make a stop in Constantinople and depose the emperor. Before that, the Doge made the Crusaders stop at the rich city of Zara and sake it, destroying the great rival of Venice in the Adriatic Sea.
For this, Innotentius III excommunicated the crusaders. It was a Crusade no more, now it was just a mercenary army. They deposed Alexios III, and crowned Alexios IV. But the Greeks made a rebellion and the fourth Alexios died. Alexios V Doukas, an important officer in the Imperial Court, took the Crown and prepared the city for a siege, for he was anti-latin. But the latins entered the city (the City that never had been conquered!) due to a great fire that consumed a great part of the inner quartiers. And you know the rest.
But I don't know what's the point in putting the Byzies under the Pope's authority. The First Crusade is not a reason. It's true that parts of the Byzantine army fought along with the crusaders, but when the crusaders rejected to return Antioch to the Byzantines, they went back to Roman territory. Besides, the call of Alexios I Komnenos for help was not the reason of the Crusade. It was an excuse. The real reason is much more complex, goes through the Investitures Contest between the Papacy and the Kaiser of the Holy Roman Empire, and the fight for the Dominium Mundi, the rule of the world. Its roots are deep in the centuries, reaching the years of Roman Emperor Theodosius and the decay of the central rule in the Roman Empire.
Former Popes had fought many wars against the German Emperor, and Urbanus II did not wanted to start another conflict. So he looked for a common enemy and found out an objective and a pretext to a sacred war. It made that all the errant knights and second sons of great dynasties went to the East, and the militar tension in Europe decreased.