How was Crusading Justified?
Crusades were wars—that is firm ground. More specifically, crusading was represented as just warfare, according to the idea of just war established by St. Augustine of Hippo.
Augustine wrote that warfare was sometimes a necessary and lesser evil in certain conditions. Specifically, a war could be just if there was:
a) just cause, b) legitimate authority, and c) right intention.
Throop, Susanna A., "How was Crusading Justified?" (2014). History Faculty Publications. Paper 4.