Introduction
This report examines how moral, legal, and operational frameworks intersect when states decide to wage war and conduct it. It begins by clarifying just war theory’s core distinctions—jus ad bellum (justice of going to war), jus in bello (justice in conduct), and jus post bellum (justice after war)—and the criteria that govern each. It then explores how U.S. constitutional war powers align with these moral requirements, especially in allocating authority to initiate and manage conflict. Finally, it shows how jus in bello principles are translated into concrete Rules of Engagement that shape targeting, proportionality, and protection of civilians in modern operations.
Just war theory arises from the tension between rejecting war as a moral disaster and accepting that, at times, failing to use force can allow grave injustice to persist [1][5]. The tradition has developed into a structured framework that evaluates war across three stages: the decision to go to war (jus ad bellum), the conduct of war (jus in bello), and responsibilities after conflict (jus post bellum) [1][3][4][5]. This tripartite structure now shapes both moral analysis and legal practice, including U.S. constitutional war powers and the rules of engagement (ROE) that govern soldiers in the field.
At the threshold of force, jus ad bellum sets criteria for when going to war can be morally justified. Just cause demands that war respond to serious wrongdoing—most paradigmatically self-defense or defense of others against aggression—rather than power projection, conquest, or resource acquisition [2][5]. Right intention requires that even when a just cause exists, leaders pursue war for the sake of a just peace, not for revenge, prestige, or domestic political gain [2][5]. The principle of legitimate authority restricts the power to initiate war to recognized political institutions, which in the U.S. context focuses attention on the allocation of war powers between Congress and the President [4][5]. Just war theory’s insistence on “proper authority” resonates with Article I’s Declare War Clause and the use of Authorizations for Use of Military Force (AUMFs), raising normative questions about whether unilateral executive uses of force satisfy the requirement of collective, accountable authorization [3].
Two further ad bellum conditions highlight the moral burden of deciding for war. Reasonable probability of success forbids launching or prolonging war when achieving the just objectives is unrealistic, on the grounds that it would sacrifice lives without sufficient hope of securing the intended goods [2][5]. Last resort calls for serious, good-faith efforts to exhaust nonviolent measures—diplomacy, negotiation, sanctions, and other tools—before resorting to arms [5]. Overarching these is proportionality at the strategic level: the overall harms of war must not outweigh the anticipated goods, and the peace achieved must be meaningfully better than both the status quo ante and the foreseeable consequences of not fighting [3][4][5].
Historically, these moral ideas interacted with the law of nations and shaped how early U.S. legal actors thought about war powers. Founding-era understandings treated “just war” broadly: any grave violation of rights—including treaty breaches—could count as a just cause for war [3]. Because such violations could legally justify hostilities, Congress’s power “to declare war” was understood to carry incidental authority over matters likely to provoke or justify war, such as modifying or annulling treaties [3]. This linkage underscores a conception of Congress as a primary moral and legal gatekeeper, consistent with the just war requirement that legitimate authority carefully weigh causes for war rather than leaving them to unilateral executive discretion.
Once war begins, jus in bello governs how force may be used, maintaining a crucial conceptual barrier between the justice of the cause and the justice of conduct [1][3][4][5]. A war can be unjust in its origins yet still be fought with relative restraint, or initially just yet become unjust through disproportionate or indiscriminate violence. The central in bello principles are discrimination, proportionality, and military necessity, along with the humane treatment of prisoners and the protection of civilians [2][3][4].
Discrimination requires distinguishing between legitimate military targets—combatants and certain military objects—and those who must not be directly targeted, such as civilians and persons hors de combat [2][3][4]. Proportionality at the operational and tactical levels demands that anticipated civilian harm not be excessive in relation to the concrete and direct military advantage expected from an attack [2][3][4]. Military necessity permits only that degree and kind of force required to achieve a legitimate military objective; it forbids wanton destruction or attacks that offer negligible military value relative to their human cost [1][2]. These overlapping concepts are embedded in the modern law of armed conflict and international humanitarian law, and they form the moral backbone of contemporary ROE.
ROE translate jus in bello from abstract principles into concrete guidance for commanders and soldiers. They specify who may be engaged under what circumstances, what weapons or tactics are authorized, and what approval thresholds apply to targets with potential for collateral damage. The requirement of discrimination manifests in target identification standards, positive identification (PID) rules, and restrictions on attacking dual-use infrastructure or forces mingled with civilians. Proportionality is operationalized through collateral damage estimation methodologies, tiered approval processes for high-risk strikes, and standing prohibitions or heightened scrutiny for attacks likely to affect protected sites such as hospitals or cultural property. Military necessity appears as the demand that every use of force be tied to a legitimate military objective, with ROE and targeting doctrine requiring commanders to show that an attack contributes meaningfully to the campaign rather than merely inflicting damage.
In practice, the line between jus ad bellum and jus in bello can blur, especially around proportionality. Strategically, proportionality features in assessing whether the goals of the war justify initiating or continuing large-scale hostilities; tactically, it governs whether a particular strike is justified given specific civilian risks [1][2][3]. Commanders are thus forced to reason at multiple levels at once: the campaign must be proportionate overall, and each action within it must satisfy proportionality and necessity constraints. Additionally, emerging attention to jus post bellum feeds back into targeting decisions: choices about striking electrical grids, water systems, or other critical infrastructure increasingly consider not only short-term military gain but also the impact on post-war reconstruction, governance, and civilian welfare.
Jus post bellum itself has become a distinct, though less settled, branch of just war thought, emphasizing justice in peace terms, reconstruction, reconciliation, and accountability after fighting ends [1][3][4][5]. It raises questions about who bears obligations to rebuild, compensate, and secure a stable, rights-respecting order, and it insists that the resulting peace be superior to both the pre-war status quo and the alternatives available during conflict [1][3][4]. Unlike jus ad bellum and jus in bello, there is no clear constitutional analogue dedicated specifically to jus post bellum responsibilities, even though Congress controls funding, long-term deployments, and peace-related legislation. This institutional gap highlights a broader problem: existing U.S. checks and balances more clearly structure the decision to go to war and the regulation of military operations than they do the moral duties of reconstruction and post-conflict justice.
Across all three stages—whether in the constitutional assignment of war powers, in the moral appraisal of causes for war, or in the operational detail of ROE—just war theory functions less as a mechanical formula and more as a demanding evaluative framework. It must be interpreted amid evolving forms of warfare, including civil wars, asymmetric conflicts, and technologically mediated operations such as drones and cyber capabilities [1][3][4]. Yet despite changing contexts, the core commitments remain: constrain the resort to war to cases of serious wrong addressed by legitimate authority; constrain conduct in war through discrimination, proportionality, and necessity; and shape the end of war toward a just and durable peace.
Conclusion
Taken together, these sections show that “just war” is not a slogan but a demanding moral framework that runs from the first debate over force to the last decisions about peace. Jus ad bellum clarifies when war may be initiated and highlights how U.S. constitutional war powers can, but do not always, embody requirements of just cause, right intention, legitimate authority, and last resort. Jus in bello and jus post bellum then carry those commitments into battlefield conduct and postwar responsibility, with rules of engagement serving as the practical hinge translating abstract principles into concrete restraints on violence.
Sources
[1] https://en.wikipedia.org/wiki/Just_war_theory
[2] https://www.learnreligions.com/just-war-theory-p2-250987
[3] https://iep.utm.edu/justwar/
[4] https://www.carnegiecouncil.org/explore-engage/key-terms/just-war
[5] https://ethics.org.au/ethics-explainer-just-war/
[6] https://en.wikipedia.org/wiki/War
[7] https://yalelawjournal.org/pdf/e.2394.Hessel.2444_q8vx7dyh.pdf
[8] https://emergingcivilwar.com/2025/07/14/just-war-theory-and-the-american-civil-war/
Written by the Spirit of ’76 AI Research Assistant
Realizing News 
Leave a comment