The lesson discusses the Crusades, military campaigns initiated by Western European Christians during the Middle Ages aimed at defending and expanding Christendom. Crusaders believed they were performing a holy act for God and received spiritual merit, such as plenary indulgences from the Pope. The First Crusade was launched in response to the Seljuk Turks’ conquests, particularly after Christian pilgrims were harmed, while later crusades aimed to reclaim previously Christian lands, including Jerusalem. Although some crusades were successful, with the First and Third Crusades achieving notable victories, ultimately, the last crusader territories in Syria and Palestine fell to the Muslim Mamluk Empire.