The reason for the middle eastern crusades was a war between Christians and Moslems which centered around the city of Jerusalem. The City of Jerusalem held a Holy significance to the Christian religion. The Church of the Holy Sepulchre in Jerusalem commemorated the hill of crucifixion and the tomb of Christ’s burial and was visited by Christian Pilgrims.
Even though for 400 years the spread of Islam had diminished Christianity in the middle east, parts of Eastern Europe and was making inroads into the Byzantine Empire, the Saracen Caliphs that held Palestine and Jerusalem usually pursued an enlightened policy toward the pilgrims, even encouraging them as a source of revenue.
But in the 11th Century the Seljukian Turks wrested most of the Caliphs Asian possessions from them. And in 1065 Jerusalem was taken by the Turks and 3,000 Christians were massacred, churches were destroyed or used as stables and unlike the previous rulers, pilgrims were mistreated.
The Turks had been making constant advances in the East, and were now threatening Constantinople itself. The Greek emperor (Alexius Comnenus) sent urgent letters to the Pope, asking for aid against the infidels, representing that, unless assistance was extended immediately, the capital with all its holy relics must soon fall.
All these actions aroused a storm of indignation throughout Europe and awakened the desire to rescue the Holy Land from the grasp of the “infidel.” Starting a chain of events which contributed to the cause of the first crusade.
The first crusade was successfull and the ‘Latin Kingdom of Jerusalem’ lasted for almost 200 years (1099-1291). Though it lost Jerusalem in 1187 after the Battle of Hattin to Saladin, the ‘Latins’ re established in Acre and survived until 1291 when it was destroyed by the Egyptian Mamluks.
All up there were 9 crusades to the middle east. Throughout their history some were successfull, most were failures.