Bellum Justum, or "Just War," is a philosophical and ethical concept that outlines the conditions under which a war can be considered morally justifiable. It originated in ancient philosophy and was further developed by thinkers like St. Augustine and St. Thomas Aquinas. The theory emphasizes that a war must be fought for a just cause, such as self-defense or protecting the innocent, and must be declared by a legitimate authority. Additionally, Bellum Justum includes principles that govern the conduct of war, known as jus in bello. These principles advocate for proportionality and discrimination, meaning that combatants should avoid unnecessary harm to civilians and use force that is proportional to the threat faced. This framework aims to limit the suffering caused by war and promote ethical conduct among nations.