Urban II had been Pope for seven years in 1095. But the events he is best remembered for had their origin in March of 1095, when an ambassador from from the Emperor of Byzantium called upon him for aid against the Turk, who had captured much of the Anatolian hinterland and would soon press upon Byzantium itself.
At the Council of Clermont in November 1095, Urban called for a Crusade to retake the Holy Land (Palestine) from the Muslims. This would both place Jerusalem in Christian hands and relieve pressure on Byzantium by opening up another front in its war. What would become known as the First Crusade (of Nine) started the following year, in 1096, and lasted (in its active phase) until 1099. It was the most geographically successful of the Crusades, but its longest term effects were the reopening of trade between Europe and the Levant (and by extension, to its trading partners beyond) and the importation of Arabic texts (some of them translations of Greek and Roman texts) that led to the scientific revolutions of the next thousand years.
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