Overview of Just War Theory
The Just War Theory is a concept in the philosophy of war that addresses how wars are fought and what makes a war justified, including conduct during and after a martial conflict. Traditionally, the Just War Theory is divided into three parts: jus ad bellum, jus in bello, and jus post bellum. That is, what is required for a state to declare war, how states should conduct themselves in war, and how states should handle the terminal phase of a conflict.
While much of the Just War Theory may seem clear-cut, advancements in technology and changes in military tactics have created aspects that challenge traditional interpretations—such as destroying supply lines, the use of drones, chemical, biological, radiological, and nuclear (CBRN) warfare, and information warfare.
Moseley explains in the Internet Encyclopedia of Philosophy article “Just War Theory” that the concept has existed almost as long as humans have engaged in conflict (2). Some of the earliest thinkers to write on the Just War Theory include Francisco de Vitoria (1486–1546), Francisco Suarez (1548–1617), Hugo Grotius (1583–1645), Samuel Pufendorf (1632–1704), Christian Wolff (1679–1754), and Emerich de Vattel (1714–1767) (Moseley 3).
Moseley notes that early discussions of Just War Theory focused mainly on the Christian perspective, as seen in the writings of Saint Thomas Aquinas, or even earlier in the Bible and Homer’s works. However, the theory experienced a revival in the late 20th century due to the rise of nuclear warfare, deterrence strategies, and post-9/11 thinking (3).
Jus ad Bellum
Jus ad bellum refers to the requirements for a country to justify going to war. It is generally divided into six criteria: just cause, last resort, proportionality, proper declaration, right intention, and probability of success (Moseley 6).
Just cause is arguably the most important criterion. War destroys natural resources, tears apart families, and causes great loss of life. Therefore, states that engage in war must have a morally sound reason, such as self-defense against unprovoked aggression (Moseley 6).
Last resort, also called necessity, means war should be undertaken only when all other diplomatic and peaceful options have been exhausted. If alternatives exist that could prevent war with less destruction, they should be pursued first. Only if these fail should war be considered (Lazar 12).
Proportionality must be distinguished between ad bellum and in bello contexts (Lazar 12). As Lazar states, “The central task of the proportionality constraint… is to identify reasons that tell in favour of fighting and those that fight against it” (12). Actions must be weighed against the harm they will cause and the potential good they could achieve.
Proper declaration requires that war be announced by a legitimate authority in accordance with international law. This can be difficult to determine in conflicts where some factions are not internationally recognized. For example, during a civil war, a faction may meet all other requirements for war but lack recognized authority to declare it.
Right intention means the war must be waged for justice rather than self-interest or expansionism. A war to seize land, exploit resources, or impose an undemocratic government cannot claim right intention (Moseley 7).
Probability of success requires that war has a reasonable chance of achieving its desired outcome. If success is unlikely, war will only prolong or worsen the situation (Lazar 6).
Jus in Bello
Jus in bello concerns how states should conduct themselves during war. Traditionally, it is based on three principles: discrimination, proportionality, and necessity (Lazar 12).
These principles distinguish between combatants and non-combatants. While combatants may be targeted, harm to non-combatants must be avoided whenever possible, and when unavoidable, it must be unintentional, proportionate, and necessary (Lazar 13).
In practice, the distinction is not always clear—especially in asymmetric warfare or when civilians assist combatants. Infrastructure, land, and resources generally are not legitimate targets under jus in bello, but ambiguity arises if these directly support military efforts (e.g., farms feeding soldiers, munitions factories staffed by civilians).
Other tactics that exist in a moral gray area include CBRN attacks, targeted assassinations, landmines, hostage-taking, and destroying supply lines that serve both civilian and military needs (Moseley 13). In these situations, proportionality and discrimination are key.
Jus ad bellum and jus in bello are interconnected. As Moseley writes, “Responsibility for acts of war relates back to the tenets of jus ad bellum as well as jus in bello” (14). If a war is not just to begin with, it is unlikely to be fought justly.
Difficulties arise when one side ignores the Just War Theory or the rules of engagement. While the opposing side may attempt to follow the rules, they may be disadvantaged when fighting an enemy that disregards them. Such situations can escalate conflicts and cause global instability.
Jus post Bellum
At the end of a conflict, three outcomes are possible: one side wins, one side loses, or a ceasefire is reached. Jus post bellum addresses how parties should conduct themselves at this stage.
Its principles include avoiding wrongful punishment of civilians and ensuring that reparations and penalties are fair and proportionate (Moseley 14). The winning side often imposes its agenda on the losing side, but rehabilitation is essential to prevent future wars.
Rebuilding the losing side promotes economic growth, diplomatic cooperation, and alliances that can prevent future conflicts. Actions taken during war should be considered in light of their post-war consequences. For example, weapons of mass destruction can leave long-term devastation—raising questions about rebuilding responsibilities. The same applies to chemical weapons that render land unusable for generations.
As technology evolves, so too must the Just War Theory. With autonomous weapons, drone strikes without direct human decision-making, and cyberattacks targeting infrastructure and economies, the rules of war must adapt.
Works Cited
Moseley, Alexander. “Just War Theory | Internet Encyclopedia of Philosophy.” Internet Encyclopedia of Philosophy, iep.utm.edu/justwar/#H2.
Lazar, Seth. “War.” Stanford Encyclopedia of Philosophy.
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