The Crusades are remembered unrealistically and romantically as a business of knights on a sacred mission. There is no concept in Christianity of a holy war — the Crusades were essentially pilgrimages that included fighting.
Pope Urban II instigated the 1st Crusade in 1095 to liberate Jerusalem from the Seljuk Turks, who treated Christian pilgrims harshly. By the time the Crusaders arrived, the Shia Muslim Fatimid Egyptians had taken Jerusalem; the Sunni Turks did not help them and the Crusaders prevailed.
In 1187 Saladin took Jerusalem back and Pope Gregory VIII proposed a 3rd Crusade. This involved the great soldier, King Richard I.
By the 4th Crusade, the concept had degenerated. Of the 35,000 volunteers, only 11,000 reached Venice; fighting then oscillated from one exploit to another, depending on who could pay; and they were excommunicated for attacking Christians.
The Crusades were not successful in their own terms and were a major drain on resources.