What effects did the Crusades have on the Islamic world and Byzantium?
The Crusades began in answer to an urgent request from Byzantine Emperor Alexius Comneus to Pope Urban II to rid the area of Muslim Turks. Most scholars agree that the Crusades slowed the advance of Islam, with the presence of Crusader states in the Near East forming a barrier between the Arabs and Turks, preventing the formation of a unified Islamic power. Because effort had to be diverted to defense, it slowed the pace of conquest.
A side effect of the Crusades was new knowledge of the East, with possibilities for trade. However, this also encouraged additional violence, conquest, and bloodshed as religious interests combined with secular and military enterprises. This includes the belief that the military conquest of Constantinople actually hastened the fall of the Byzantine Empire, as it was never able to regain its former strength. Also, with new knowledge of trade, economic trends shifted as well. Commercial trading shifted from Muslims to Italians in the Mediterranean, with a gradual shift to the Atlantic, and Spain and Portugal seeking new trade routes and opening the world up to additional exploration.