During the Medieval period, Europe was weak, especially when compared to the important Islamic states to the east and south of them. Another increasing problem for Christendom was the encroaching Viking pagans from the north, who were not only taking significant chunks of northern Europe, but actually invaded into eastern Slavic lands, Italy, and northern Africa. When the two powers surrounding Europe began to enter into trade and military agreements, Pope Urban II decided that he needed a way not only to get rid of the Muslims (the Norse were slowly being converted), but also of religious dissidents at home. His solution, in 1095 C.E., was to begin the Crusades, obstensibly to "liberate" the sacred Palestinian area (and earn the fighting rabble a free ticket into Heaven) but also to attempt to usurp trade routes between the Norse and the Muslims, and between the Muslims and far eastern Asia.
Although the first crusade was a military fiasco, the next few were financially successful. By the fourth, the Doge of Venice actually financed a crusade to sack Byzantium, the Eastern Orthodox Christian capital! Of course, such a campaign could not last forever, and by the Renaissance such goings on were replaced by the financially and militarily more successful Inquisition. By this time, though, Islamic learning and goods had made their impact upon Europe. Indeed, one could make a strong case that the European Renaissance was only possible with the influence of their enemy, the Muslims.