The first law of history, is not to dare to utter falsehood; the second, not to fear to speak the truth.
Pope Leo XIII
During the Fourth Crusade in April 1204, Crusaders from Western Europe, on their way to the Holy Land "to re-claim Jerusalem for Christ" from the Muslim occupation, stopped off in Constantinople and besieged the city, stealing, burning, or destroying as they scoured the capital. They looted the churches of relics, art, sacred vessels, statues and murdered tens of thousands of ecclesiastics and laymen alike. The high altar in Hagia Sophia was smashed to bits and crusader knights carried off a piece as a souvenir of the desecration. The occupation lasted sixty years and was the real reason the Byzantine Empire declined to the point in 1453 when it was finally overcome by Islam. Even after they were repulsed from Constantinople, the invaders held pockets and islands of the empire, destroying its unity and making it doubly vulnerable to absorption by the Muslims.
The plot to use the crusades as a venture of rapine was developed in the main by European nobles, with the Venetians in the lead. They saw Constantinople and the Byzantine Empire as a jewel waiting to be plundered. It never really recovered from this vicious attack by fellow Christians. The capital Constantinople was occupied for over sixty years, and the Muslims took advantage of this and continued their advance by swallowing most of the former lands attached to the Empire, slowly choking off its Balkan Peninsula Commonwealth and the lands in what is now called Turkey. The Turks finally surrounded the city of Constantinople, New Rome itself. The Venetians had spearheaded the plan to destroy the Byzantine Empire, as it was their principal maritime commercial competition for Mediterranean naval trade route operations.
The Second Council of Lyon (1272-1274) was presided over by Pope Gregory X and was convened to act on a papal call for another crusade to recapture the Holy Land and a pledge by the new Byzantine Emperor Michael VIII Palaiologos to reunite the Eastern Church with the West. He had regained control of Constantinople in 1261 from the Latin invaders and wanted to cement his position by making peace with the Roman Church as well. Hoping to end the Great Schism that divided the Churches, Gregory had sent an embassy to Michael. He also asked Latin despots in the East to hold their ambitions. The ambassador of the Emperor along with members of the Greek clergy arrived at Lyon on 24 June and presented three letters from Michael. On 29 June (the Feast of Sts. Peter and Paul and patronal feast of the popes), Gregory celebrated a Mass in St. John's Church, where both sides took part. The Greeks read the Nicene Creed, with the Western...