Jerusalem was important to three different religions. The Jews had a temple there. The Christians were interested in Jerusalem because Jesus Christ taught there. The Moslems thought Jerusalem was important because Mohammed left Jerusalem for heaven.
By the end of the eleventh century the Muslims were in control of the area around Jerusalem. The Turkish people, the Moslems, living in Jerusalem destroyed many Christian shrines and made it difficult for Christians to make pilgrimages there.
At this time the Byzantines were having trouble fighting the Turks. They asked Pope Urban II for help. In 1095 the Pope held a meeting. He said that the Christians should free Jerusalem. He further said that killing Moslems was much better than killing each other at home. He stated that if a person died fighting Moslems his sins would be forgiven.
After Pope Urban II spoke thousands of men went to fight this Holy War. These people were mostly peasants and townsmen. The men were lead by Peter the Hermit. The army crossed the Rhine and moved toward Constantinople. The stole everything they could along the way. The emperor sent them on to Asia. There they were killed by the Turks.
The Second Crusade was much better organized. The leaders were dukes and counts mostly from France. Four groups met at Constantinople. Again the emperor sent them across the channel. They won a few battles on the way to Jerusalem. When they reached the city they massacred the Muslims and Jews including the women and children.
States were established at Edessa, Antioch, Tripoli, and Jerusalem. These states lasted for many years. Seven other crusades took place over the next two hundred years. The most noted was the Fourth Crusade. During this crusade the Venetian talked their leaders into attacking Constantinople. They sacked the city and carried off all valuable pieces of art. The Venetian controlled Constantinople for 47 years.
The crusades eventually ended when Sultan Saladin attacked Jerusalem. He took the city and drove the Crusaders out.