The initial conquest of Palestine by the forces of Islam did not interfere much with pilgrimage to Christian holy sites such as Jerusalem, Bethlehem, and Nazareth. However, in the year 1004 the Fatimid caliph of Cairo, Hakim[?], had the Church of the Holy Sepulchre destroyed. His successor permitted the Byzantine Empire to rebuild it, and pilgrimage was permitted again.
In Western Europe the Crusades have traditionally been regarded by laypeople as heroic defensive enterprises, although not all historians have agreed. In the Islamic world, however, the Crusades are regarded to this day as cruel and savage onslaughts by Christendom on Islam, and so, for example, some of the rhetoric from Islamic fundamentalists uses the term "crusade" in this emotional context to refer to Western moves against them. Eastern Orthodox Christians also see the Crusades as attacks by the West, especially because of the sack of Constantinople by the Fourth Crusade.
There is an interesting symmetry between the terms "Crusade" and "Jihad". In the West the term "Crusade" has positive connotations (for example a politician might use rhetoric such as "a crusade against illegal drugs") while the term "Jihad" has negative connotations associated with fanatical holy war. In the Islamic world the term "Jihad" has positive connotations that include a much broader meaning of general personal and spiritual struggle, while the term "Crusade" has the negative connotations described above. Thus to correctly translate nuances of meaning, the use of "Jihad" in Arabic should be translated to "Crusade" in English while use of the Arabic term for "Crusade" should be translated to "Jihad" in English.
In truth much of what the crusaders did was less than heroic. They committed atrocities not just against Muslims but also against Jews and Christians. For example the Fourth Crusade never made it to Palestine, but instead sacked Constantinople, the capital of the Christian Byzantine Empire. This crusade served to deepen the already hard feelings between Eastern Orthodox Christianity and Western Christianity. The Byzantine Empire eventually recovered Constantinople, but its strength never fully recovered, and the Byzantine Empire finally fell to the Ottomans in 1453.
After Byzantine Emperor Alexius I called for help with defending his empire against the Seljuk Turks, in 1095 Pope Urban II called upon all Christians to join a war against the Turks, a war which would count as full penance. Crusader armies marched up towards Jerusalem, sacking several cities on their way. In 1099, they took Jerusalem, massacring the Jewish and Muslim population. As a result of the First Crusade, several small states were created, notably the Kingdom of Jerusalem.
After a period of relative peace, in which Christians and Muslims coexisted in the Holy Land, Bernard of Clairvaux called for a new crusade when the town of Edessa was conquered by the Turks. French and German armies marched to Asia Minor in 1147, but failed to accomplish any major successes. In 1149, both leaders had returned to their countries without any result.
In 1187, Saladin captured Jerusalem. Pope Gregory VIII preached a crusade, which was lead by several of Europe's most important leaders: Richard I of England, Philip II of France and Frederick I, Holy Roman Emperor. Frederick drowned in Cilicia in 1190, leaving a unstable alliance between the English and the French. Philip left in 1191 after the fall of Acre, while Richard left the following year after establishing a truce with Saladin.
The vital crusading spirit was now dead, and the succeeding crusades are to be explained rather as arising from the efforts of the papacy in its struggle against the secular power, to divert the military energies of the European nations toward Syria.
An outburst of the old enthusiasm led to the Children's Crusade of 1212, which Pope Innocent III interpreted as a reproof from heaven to their unworthy elders. None of the children actually reached the Holy Land, being sold as slaves or dying during the journey of hunger.
By processions, prayers, and preaching, the Church attempted to set another crusade on foot, and the Fourth Lateran Council (1215) formulated a plan for the recovery of the Holy Land.
In 1228, Emperor Frederick II set sail from Brindisi[?] for Syria, though laden with the papal excommunication. Through diplomacy he achieved unexpected success, Jerusalem, Nazareth, and Bethlehem being delivered to the Christians for a period of ten years.
The later Edward I of England undertook another expedition in 1271, retiring the following year after a truce. With the fall of Antioch (1268), Tripoli (1289), and Acre (1291) the last traces of the Christian occupation of Syria disappeared.