Beginning in the 200s, Western European Christians often made pilgrimmages to the Holy Land, and into Jerusalem to retrace the steps of Jesus. Muslims conquered the area in the 600s, but tolerated the Christians. However, in 1071 the Seljuk Turks were intolerant of non-Muslims. Within a few years, the Turkish armies threatened the Byzantine Empire.
Pope Urban II was asked by the Byzantine emperor for help to turn back the Turks. In 1095 at Clermont, France, Pope Urban preached of a crusade to regain the Holy Land to the French nobles. He promised salvation to those who died on the crusade. Pope Urban hoped the crusade would help reunite the Church and lessen feudal warfare.
Thousands of people, peasants and knights alike, responded to Urban's request. Although they all headed to the same place, their motives were as different as night and day. Some went for the riches, others for salvation, some for the chance to fight for the most worthy of all causes -- for the glory of God. In 1099 Jerusalem was captured and four feudal states set up.
Palestine later fell into Muslim hands, and several more crusades took place, although none were as great a success as the first.
As a result of the crusades, Europe emerged from its shell of isolation. Trade with foreign countries grew, and several strong trading centers emerged.
Reading this section of this book on the Crusades, my understanding of this time period was increased, but I didn't really like reading it.