The Crusades caused a period of military expansion by the powers of Europe, lead by Pope Urban II and the Catholic Church. At the end of the 11th century CE, Europe was emerging from the dark ages and had demonstrated that it’s various lords and kings had significant military power. The Byzantine empire, which was formerly the Eastern Roman Empire, had been losing large swaths of territory to Muslim Seljuk Turkish forces, who had managed to capture the "Christian" Holy Land as a result. During the year 1071, the Byzantine Empire’s forces were soundly defeated at the battle of Manzikert, which allowed Turkish forces to claim control of a large portion of present day Anatolia. This defeat allowed Alexius Comnenus to take power in 1081; he was able to unite the Byzantine army under one banner.
The original idea of crusading was born when Alexius I requested support from Pope Urban II to help him counteract the invading Turks. Pope Urban decided to send forces to Constantinople, resulting in the first crusade.
The relationship between the Church and the Byzantine Empire had undergone some stress over time, but the request from Alexius came at a time when relations were steady. Because of this, Pope Urban the II urged European Christians to take up arms against the Infidels. Muslims forces possessed the holy land, and they were not permitting Christians to travel there on pilgrimage, which angered many. This factor also contributed in the motivation of many would-be Christians soldiers. As a result, the response from a large number of Christians was very positive, which lead to the armies of Crusaders proceeding on the pilgrimage during the first crusade.
Above: a video of The Crusades, explained in 5 minutes.
Above: An impression of what the Crusaders might have looked like.