Some of the negative aspects of the Crusades include massive loss of life, financial burdens that weakened the European nobility and a growth of hostility between religions. The Crusades lasted roughly from 1095 to 1270.
In 1095, Pope Urban II called for a crusade to liberate Jerusalem from the Turks. He declared that the knights of France who embarked on this quest would be absolved of all sins and escape the possibility of going to hell. Over 60,000 people responded to his call. It took four years for them to assemble and reach Jerusalem, and numbers of both knights and peasantry perished along the way.
Crusaders were motivated by the belief that the Christian God wanted them to eliminate unbelievers in order to restore prosperity to Europe. For this reason, the First Crusade is often cited as the beginning of anti-Semitism in Christians. While the crusading knights often relied on Jewish merchants for financing, crusading peasants saw Jews as an enemy of Christianity and slaughtered thousands on their journey through the Rhineland.
The 1099 conquest of Jerusalem was successful, but it also resulted in the countless deaths of both Christians and Muslims. Crusading chronicles describe the amount of blood on the battlefield as reaching as high as their horses' knees. The fundamental rift between religions created by the Crusades has been cemented throughout the ensuing centuries.
The struggle to finance the Crusades also reformed the economy of Europe. While the weakening of the nobility and the rise of the merchant class that resulted from the Crusades can be seen as positive in some ways, it resulted in the formation of a new tax system, which created a heavy tax burden for the common people.