The Crusade is believed to be the start of the relentless rivalry between the Muslims and Christians. The crusades were expeditions undertake in fulfillment of a solemn vow to deliver the Holy Places from Mohammedan Tyranny but later developed into territorial wars. The term crusader means to "mark with cross" which suites their purpose of redeeming their land.
Before the crusades, relationship between Muslim and Christians are not hostile and were mainly commercial and intellectual. They trade slaves and northern products such as honey, amber, and furs. Muslim wanted slaves but Christian teaching forbid Christians to enslave other Christians and therefore enslave pagans. This was considered a profitable relationship were it went down through central Europe to the Muslim world and some through Spain through the channels of Byzantine trade.
But as these go on, Islam population and territory expands until they reached Jerusalem. However, this did not interfere much with the pilgrimage to Christian holy site or the security of monasteries and Christian communities in the Holy Land, and western Europeans were not much concerned with some loss of Jerusalem when, they themselves faced with invasions from the Muslims.
Muslims..
The Holy Seapulchre Church, Jerusalem. Catholic Ho...
English: Medieval miniature painting of the Siege ...A turning point in western attitude came in the year 1009 when the Fatimite Caliph of Egypt, in a fit of madness ordered the destruction of the Holy Sepulchre and all Christian establishments in Jerusalem. For years thereafter, Christians were cruelly persecuted.
Much of the reasons of the crusade were stated by Pope Urban II's proclamation of the first crusade on November 25, 1095 at the Council of Clermont:
Addressing a vast crowd of priests, knights and poor people. The Seljuk turks, he explained, a barbarian race from central Asia who had recently become muslims, had swept into Anatolia in Asia minor(now turkey) and had seized these lands from Christian...