Just war theory is a philosophical framework that seeks to determine the moral justification for engaging in war. It outlines criteria that must be met for a war to be considered just, including a just cause, legitimate authority, and proportionality. This theory helps to evaluate whether the reasons for going to war are ethically sound and whether the means used in warfare are appropriate. The theory is often divided into two main parts: jus ad bellum, which addresses the justification for entering a war, and jus in bello, which focuses on the conduct of war. Key figures in the development of just war theory include St. Augustine and Thomas Aquinas, who emphasized the importance of moral considerations in warfare.