The Crusades were a series of military campaigns during the time of Medieval England against the Muslims of the Middle East. In 1076, the Muslims had captured Jerusalem. Jerusalem was the most holy of holy places for Christians. This is because Jesus had been born in nearby Bethlehem and Jesus had spent most of his life in Jerusalem. He was crucified in Jerusalem on Calvary Hill. Jerusalem was the most important place on Earth for a true Christian which is why Christians called Jerusalem the "City of God". However, Jerusalem was also extremely important to the Muslims also. This is because Muhammad, the founder of the Muslim faith, had been there and there was great joy in the Muslim world when Jerusalem was captured. A dome, called the Dome of the Rock, was built on the rock where Muhammad was said to have sat and prayed and it was so holy that no Muslim was allowed to tread on the rock or touch it when visiting the Dome. As a result, the Christian fought to get Jerusalem back while the Muslims fought to keep Jerusalem. These wars were to last nearly 200 years.
The jihad is often translated as "holy war on behalf of Islam". To them if a war is for the will of Allah then it is right to fight. This is one way inwhich the idea of jihad influenced the crusades. The Islams thought of the crusades as holy wars. Therefore they agreed to fight in them. (Internet Book)
Most Muslim Scholars see the world as divided into two houses, the House of Peace and the House of War. To them the lands controlled by Muslims belong to the House of Peace, while those who have not yet submitted to Islam belong to the House of War until they are subdued.
So the entire context of the eastern Crusades is one of response to continuous Islamic aggression.
"In the year 1095, people were shocked in Western Europe by the words of Pope Urban II, "The Muslims have conquered Jerusalem". Pope Urban wanted the Christians to retake Jerusalem from the Muslims. People shouted "God wills it". . . Religion was important to the knights in the Middle Ages. One of the results of the Crusades was the founding of new Christian religious orders. Most of the monks were former knights who fought against each other in the Crusades. The knights did capture Jerusalem for a short period of time, but the Muslims kept on re-taking Jerusalem. The knights gained temporary power, but lost many soldiers during the deadly Crusades, not to mention causing the death of many innocent Muslims. The Crusades is a violent reminder of the greed of Middle Ages."
"Unlike Islam, Christianity had not yet developed the notion of a holy war. In the fifth century Augustine described what constituted a just war but excluded the practice of battle for the purpose of religious conversion or to destroy heretical religious ideas. Leaders of nations might decide to go to war for just reasons, but war was not to be a tool of the church. Unfortunately, using Augustine's just war language, Popes and Crusaders saw themselves as warriors for Christ rather than as a people seeking justice in the face of an encroaching enemy threat."
The Pope called for a war of the cross. Both, the Muslims and the Christians, thought that "God" was on their side. However, both sides can not be right. (Internet Book)