The Crusades began with an army of peasants headed by Peter the Hermit (a French monk and prominent commander of the Crusades). The Turks slaughtered them all. In 1099, an army of knights commanded by Godfrey of Bouillon (a French knight and a commander of the first Crusades) murdered Muslims and conquered Jerusalem. The Crusaders slaughtered Muslims until the streets were scarlet with blood, according to legend.
The Pope desired to unify Christians from the West and the East under his rule. With the support of Venice, he diverted the Crusade and seized Constantinople in 1204. Christians fought each other.
Christian knights suffered a devastating military loss at the hands of Ottoman Turks, putting an end to enormous international attempts to stop Turkish expansion into the Balkans and central Europe.