The Crusades were military expeditions organized by western European Christians to keep in check the spread of Islam and recapture former Christian territories that now were Muslim. The Crusades began in 1095 and lasted for almost 200 years.
To some historians, even when these religious wars presented gruesome results, they ultimately were a factor in European civilization development as the growth of the system of indulgences and the reinforced link between Western Christendom, feudalism, and militarism, led to the Protestant Reformation.