Just war theory is a set of philosophical doctrines that determine the justifiable causes for war, codes of conduct during combat, and rules for post-war actions. The theory includes jus ad bellum, jus in bello, and jus post bellum, which focus on just causes for war, codes of conduct during combat, and proportionate actions towards defeated nations after the war, respectively. The theory aims to ensure that wars are fought fairly and not undertaken lightly.
War is the most drastic means of resolving conflicts between two or more nations; therefore, certain rules and codes of conduct are put in place to ensure that the war is fought fairly and is not undertaken lightly. Collectively, all the philosophical doctrines on how and why war is fought are known as just war theory. The theory comes from the just war tradition, which originates in war between like-minded nations who agree on a mutual code of conduct. Three main components make up the theory: jus ad bellum, for the causes of war; jus in bello, for conduct during the war; and jus post bellum, for the post-war period.
The first part of just war theory, jus ad bellum, determines the justifiable causes for a nation to act as an aggressor in a war. Such reasons include having a just cause, resorting to war as a last resort after all peaceful means of conflict resolution have failed, and having good intentions. In any case, the actions taken should be proportionate to the cause. Just cause of war does not include acts of vengeance for previous actions. The only commonly accepted justification for war is defense against physical attack or expansion into the territory of the offending nation.
In times of war, the policy of jus in bello applies. This part of just war theory focuses on detailing the codes of conduct to be observed during combat; typically, the two areas covered are discrimination and proportionality. These two concepts refer to which parties are considered legal combatants and what actions can be taken against them. The Hague Court and the Geneva Conventions make up most of the jus in bello doctrine of modern times. Examples of the doctrine include expectations that civilians must not be targeted, that combatants must be given the opportunity to surrender, and that the use of chemical or biological weapons is prohibited.
After the war is over, certain rules are enforced under the jus post bellum. Just war theory requires that actions towards defeated nations in the aftermath of war be proportionate to the extent of the war and should not affect civilian lives. An example of a jus post bellum being violated is the Treaty of Versailles after World War I, where Germany was obliged to pay all reparations for the war despite being only one party to it. The treaty caused the German economy to plummet, leading to Hitler’s rise to power and World War II.
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