The Doctrine of Discovery is a legal and religious concept tied to the Roman Catholic Church which rationalized and 'legalized' colonization and the conquering of Indigenous peoples in the eyes of Christianized Europeans. The roots of the Doctrine go back as far as the fifth century popes and leaders in the church who had ambitions of forming a global Christian commonwealth. The Crusades (1096-1271) were based on this ambition of a holy war against who the church saw as "infidels". Pope Innocent IV's writings from 1240 were particularly influential. He argued that Christians were justified in invading and acquiring infidel's lands because it was the church's duty to control the spiritual health of all humans on Earth.