How did Christian ideas influence early European rules about conduct in war?

During the Middle Ages, Christian ideas significantly influenced European conduct in war by introducing moral frameworks that sought to limit violence, even if their practical application was often inconsistent.12

Key influences included:

• Just War Theory: Formulated by theologians like St. Augustine and St. Thomas Aquinas, this doctrine attempted to distinguish between "just" and "unjust" wars. It established that force should be used only for a just cause, such as defending the common good, and by proper authority.32
• The Truce of God: This Roman Catholic Church initiative mandated the suspension of warfare during specific holy days, seasons like Lent and Advent, and certain days of the week to protect noncombatants and limit the duration of conflicts.1
• Chivalry: Rooted in the knightly class, the code of chivalry incorporated Christian values of honor and courtesy. It encouraged restraints on the means of war and contributed to the emerging concept of noncombatant immunity, though these protections were often limited to the aristocracy and did not extend to the common soldier or civilian.324

While these efforts provided early guidelines, they were frequently undermined by feudal power struggles and the lack of uniform enforcement.12