The Muslims began to threaten Western Europe in the 7th century. They started as a movement in Arabia, and now they planned to conquer the whole world starting with Jerusalem. It was not until 1074 that Gregory VII started mentioning crusading against the Muslims in Jerusalem. However, he did not start the crusades due to political unrest with some of the HRE kings.
Earle Cairns declares that although there were political and economic interests, the Church really did start crusading for religious reasons, and I believe it. Islam became more and more aggressive. Seljuk Turks moved into Israel's land. Urban II preached and caused the First Crusade in 1095. This was from a cry for help from the Eastern empire, and more importantly, to rescue the Holy Land. Mostly French people responded to escape famine and feudal life.
They achieved their goal. They recaptured Constantinople, Antioch, and Jerusalem. This also saw the rise of the Knights Templars and Hospitallers who helped aid the crusaders.
The Second Crusade followed when Islam threatened northeastern Jerusalem and after the Muslims had captured in Edessa. Bernard of Clairvaux, a man I greatly admire, preached this crusade. Sadly, this one failed and the Muslims recaptured Jerusalem.
The Third Crusade, the King's Crusade, arose under Philip Augustus, king of France; Richard of England, and Emperor Frederick. Frederick drowned on the way to Israel, and the kings of France and England quarreled. Richard continued the crusade. He did not take back Jerusalem, but Saladin, the leader, did allow pilgrims to come.
Innocent III preached the Fourth Crusade and captured Egypt so that they could bring the Greek Church and Eastern empire under the pope again. This only succeeded in weakening the East and causing more hatred between the two halves of the empire.
Crusade Six, Frederick II negotiated with the Muslims who agreed to give Jerusalem, Bethlehem, Nazareth, and part of the sea to the Christians while the Muslims kept the Mosque of Omar. The lands later fell again to the leadership of the Saracens.
In 1212, the Children's Crusade caused many children, led by boys named Stephen and Nicholas, to march to Italy to amend for their parents' failure. Many of them died or were sold into Egyptian slavery. Acre fell to the Muslims in 1291, ending the crusade era.
As a result of this, feudalism weakened. Knights sold lands to the peasants to raise money for the trip, and cities began to form with a middle class, strong monarchies, and security.
This also enhanced the prestige of the pope, but also created a patriotic high that did not last long. Raymond Lull learned Arabic and Arab culture so that he could be a missionary to them. This began a Renaissance of learning Arabic science and literature. Scholastics tried to synthesize Arabic learning with Christian revelation. This did enrich the culture but also led to more compromise with non-believers.