To many people, “just war” sounds like an oxymoron. How can the lethal violence unleashed in war ever be just? Yet realpolitik underscores that not all international conflict lends itself to peaceful remedies. The twenty-first century, like many centuries before, has been one of great conflict. The U.S.-Iran war and the Russia-Ukraine war are just two examples among many current international conflicts.
Politicians often create flimsy justifications for war, but notable thinkers dating from the ancient world, like Cicero and Plato, weighed in on the matter of when and how war might be justified. Early attempts at codification of the theory’s formal framework began with St. Augustine of Hippo in the 5th century.
Later, the 13th-century philosopher St. Thomas Aquinas, in his Summary of Theology, advanced the principles of just war. His foundational work established doctrines of the concept, divided between the right to resort to war and the proper way to fight once war is underway. Hugo Grotius, “the father of international law,” in his early-17th-century work On the Law of War and Peace, initiated the reshaping of just-war theory (its basis in moral, civil reciprocity) from the theory’s theological roots into secular terms. Much later, in the twentieth and twenty-first centuries, the doctrines of just war expanded further to include a category for how parties should act once war has ended.
Through this historical development of the concept, principles governing going to war (the right to resort to war) came to include the following: the authority of qualified national rulers to declare war; just causes, such as self-defense, the defense of allies, or intervention against the gravest forms of inhumanity; right intention, meaning a sustained focus on the just cause rather than material gain, expansionism, or intemperate militarism; judiciousness, in which anticipated goods outweigh anticipated harms; a high probability of success; and war as a last resort, undertaken only after sincere negotiation and genuine attempts at peaceful resolution.
Principles governing the fighting of wars came to include these: discernment, exercised by targeting only combatants and military sites, not civilians or captives; proportionality, such that injury to lives and property aligns with the applicable military advantages; accountability, such that participants comport themselves morally and humanely; and necessity, such that the least harmful military means and force are applied relative to objectives.
Finally, principles governing behavior following war’s end and transition to peace include: restraint, where peace terms are reasonable and not the result of vengeance-seeking; fairness, where the victor resists treating the vanquished parties unduly partially, punitively, or vengefully; restoration, to promote stability, redevelopment, institutional reinstatement, self-governance, and legal order; and accountability, to determine culpability and ensure that retribution is respectful, reasonable, and measured, taking into account traditions.
Today, modern warfare has advanced technologically and become far more versatile, fast, and lethal. This dynamism demands that the principles of just war be revisited. Relevant tactics include economic sanctions, cyberwarfare, aggression through proxy forces, asymmetric fighting, special operations, psychological operations, and mercenary insertion. Individually and collectively these tactics exacerbate control over risk, behavior, and outcomes associated with warfare. Just war theory has matured from a matter of concern for nation-states and merged into the purview of human rights watchdogs and systems for international law, adding complications at every interval along the way.
These dynamics notwithstanding, international law and protocols such as the aging and limited Geneva and Hague conventions have only weak enforcement power. The unbridled messiness and “fog and friction” of wars over recent decades have demonstrated this point, underscoring the need to bind assenting nations, perhaps in profoundly new, prescriptive ways that esteem human life.
History’s chroniclers are watching. The long history of just war theory must adapt to the demands of the day or face becoming obsolete. The abruptly changing landscape of modern warfare exemplifies the reason that humankind needs a growing and developing theory of what counts as justice in warfare. We need informed, careful theorists and experts to chart the path forward to articulate what may be rightly described as ‘just war.’
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