Religious wars between Christians and the Muslims.Pope Urban II’s told the Christians to go to war against Muslim forces in the Holy Land. After the First Crusade had achieved its goal with the capture of Jerusalem in 1099, the invading Christians set up several Latin Christian states. Muslims in the region vowed to wage holy war (jihad) to regain control over the region.
http://www.history.com/topics/crusades
http://www.history.com/topics/crusades