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Customary International Humanitarian LawEdit

Customary International Humanitarian Law

Customary International Humanitarian Law (CIHL) consists of those rules that govern the conduct of armed conflict, not because they are written in a treaty, but because states across time and regions have consistently acted in ways that reflect an obligation under international law. In practice, CIHL sits alongside treaty-based provisions to create a comprehensive framework for how warfare is conducted, who is protected, and what limits apply to military force. It derives its force from two core ideas: general state practice and the belief (opinio juris) that such practice is legally required. International humanitarian law and Customary international law scholars alike treat CIHL as a binding, universal, and evolving part of the law of war.

CIHL operates in a landscape where treaties—most notably the Geneva Conventions and their Additional Protocols—codify many core norms. But customary rules fill gaps, address gaps left by treaties in lesser-covered situations, and adapt to changing technologies and forms of warfare. Because customary norms arise from how states actually behave and accept obligations, they can apply even in conflicts where specific treaties have not been ratified by every party. This makes CIHL a practical anchor for restraint when governments engage in hostilities, and it acts as a universal baseline that helps prevent a descent into absolute indiscrimination or barbarism.

Historical development and sources

The modern understanding of CIHL rests on a twofold foundation: widespread state practice and the belief that such practice is legally required. The International Committee of the Red Cross (International Committee of the Red Cross) has long played a leading role in documenting and clarifying these customary rules, most notably through the Study on Customary International Humanitarian Law. That study catalogs hundreds of customary rules, organized around core principles such as distinction, proportionality, and precautions in attack. opinio juris—the sense among states that a practice is legally obligatory—complements the observable practice, turning routine actions into legally binding rules.

Customary rules tend to codify what most states already accept as normative limits on war, even before a treaty obligates them. Over time, practice and acceptance solidify into norms that govern both international armed conflicts (where states fight states) and non-international armed conflicts (where organized groups fight within a state). The Common Article 3 to the Geneva Conventions is a widely cited example of a customary norm that has attained widespread recognition and applicability across different kinds of conflict.

CIHL does not replace treaty law; rather, it complements and fills gaps. When a treaty exists on a given topic, its provisions bind parties that have ratified it, and customary rules reflect what states accept as legally binding even in the absence of a specific treaty provision. This dual system—treaty law plus customary law—produces a more resilient, adaptable framework for regulating the conduct of war. See also Geneva Conventions and Additional Protocol I for treaty-era anchors that CIHL often reinforces through practice and opinio juris.

Core rules and concepts

CIHL rests on a set of enduring principles designed to limit harm to noncombatants and to constrain how military means are used. While much of CIHL overlaps with treaty law, its customary character matters because it binds even those who have not signed particular instruments.

  • Distinction: Parties to an armed conflict must distinguish between military targets and civilians or civilian objects. Attacks may only be directed at military objectives, and deliberate attacks on civilians are prohibited. This principle is central to both treaty and customary law and is reflected in various norms and practices recognized as customary. principle of distinction

  • Proportionality: Even when a target is legitimate, the anticipated civilian harm must not be excessive in relation to the concrete and direct military objective sought. This restraint seeks to prevent “military necessity” from becoming indiscriminate harm. See Proportionality (international law) for the broader doctrinal context.

  • Military necessity: Force used in armed conflict should be limited to what is necessary to achieve a legitimate military objective. When paired with distinction and proportionality, it helps ensure that force serves a legitimate purpose rather than sparking unnecessary suffering. See Military necessity.

  • Precautions in attack and precautions in defense: Parties must take feasible precautions to minimize civilian harm, including choosing means and methods of warfare that reduce risk to civilians. See Precautions in attack and Precautions in defense for the precise formulations in customary practice.

  • Humane treatment and protection of persons hors de combat: Civilians, wounded soldiers, and detainees must be treated humanely and protected from violence, intimidation, and torture. The rules governing this protection extend across both international and non-international armed conflicts. See Treatment of persons in armed conflict and Treatment of prisoners of war.

  • Prohibition of starvation and other methods of warfare: Denying civilians the means to survive or using starvation as a method of war is prohibited. See Starvation in war and related customary norms.

  • Cultural property and heritage: Attacks aimed at preserving cultural property are constrained to prevent culturally valuable sites from becoming targets solely for propaganda or retaliation.

  • Application to non-state actors and non-international armed conflicts: While many norms originated in state-on-state warfare, customary practice has evolved to cover organized non-state actors when an armed conflict reaches a certain threshold. See Non-international armed conflict and Common Article 3 for the baseline protections that apply in such conflicts.

These core rules are reinforced by a large body of practice documented by states, international organizations, and non-governmental organizations, with the ICRC’s studies serving as a key reference point for what is considered customary in diverse theaters of operation.

Customary IHL formation and enforcement

CIHL forms where state practice converges with the belief that those practices constitute legal obligations. This convergence can be observed in how states conduct hostilities, treat detainees, protect civilians, and respond to violations. The process by which customary norms form typically relies on:

  • General and consistent practice: Repeated, widespread behavior that states accept as appropriate in armed conflict.

  • Opinio juris: The belief that a practice is required by law, not merely preferred or convenient.

  • Documentation and interpretation: Institutions such as the International Committee of the Red Cross study patterns of practice and articulate the resulting customary rules. In turn, courts and tribunals may apply customary norms even in cases involving states that have not ratified particular treaties. See also opinio juris and Customary international law for underlying concepts.

Enforcement of CIHL occurs through a mix of mechanisms. States rely on domestic and international judicial processes to adjudicate alleged violations, while international bodies and courts—such as the International Criminal Court and related mechanisms—address accountability for grave breaches or war crimes. The presence of treaty-based enforcement (e.g., the Geneva Conventions and their protocols) reinforces customary norms, creating a robust framework for accountability. See Rome Statute of the International Criminal Court for a central instrument defining war crimes and their jurisdictional reach.

The balance between sovereignty and restraint is a defining feature of CIHL’s enforcement regime. States are free to conduct their foreign and defense policies, yet they accept limitations on the means and methods of warfare. This balance is often cited in discussions about the legitimacy of interventions, counterterrorism operations, and humanitarian concerns. See state sovereignty and rule of law for related concepts.

Contemporary debates and policy implications

CIHL remains a live field of policy debate because real-world conflicts continually test the boundaries between security interests and humanitarian constraints. From a vantage point that prioritizes strategic reliability, several themes recur:

  • Self-defense and humanitarian concerns: Critics sometimes question how far CIHL can or should constrain legitimate self-defense, particularly in fast-moving, asymmetrical confrontations. Proponents argue that CIHL provides a disciplined framework that also preserves long-term security by reducing civilian harm, avoiding vengeance-driven cycles, and maintaining international legitimacy. See self-defense and R2P (Responsibility to Protect) for related concepts.

  • Non-state actors and non-international armed conflicts: As wars increasingly involve non-state actors, CIHL’s customary rules extending to such parties are tested. The evolving practice aims to keep restraints in place even when formal treaty ratification is incomplete or inapplicable. See Non-state actor and Non-international armed conflict.

  • Drones, targeted killings, and new technologies: Advances in warfare raise questions about how traditional CIHL norms apply to precision strikes, intelligence surveillance, and remotely conducted operations. Proponents of CIHL argue that the same principles of distinction, proportionality, and precautions apply, while critics claim gaps or ambiguities persist. See targeted killing and drone warfare for connected discussions.

  • The role of R2P and sovereignty: Some debates center on whether humanitarian concerns justify intervention beyond strict legal approvals. Supporters of a robust interpretation of CIHL emphasize that this remains a rule-governed order that legitimizes responses to mass atrocities while reinforcing long-term stability. See Responsibility to Protect and state sovereignty.

  • The critique that CIHL is biased or “woke”: Critics sometimes claim that humanitarian law is a tool of Western powers or a selective standard. Proponents respond that customary norms arise from universal practice and shared humanitarian instincts, not from the preferences of any single power. They stress that many rules apply symmetrically to all parties and that the law is historically grounded in ideas of human dignity that resonate across cultures. Dismissing criticisms as simply ideological overlooks legitimate concerns about implementation, accountability, and the need to adapt rules to contemporary warfare without sacrificing core protections.

  • Implementation and accountability: A recurring policy issue is the gap between doctrine and on-the-ground practice. Training, equipment, and institutional culture matter as much as formal rules. Strong, credible accountability mechanisms—courts, audits, and transparent reporting—are viewed by many as essential to preserving both deterrence and legitimacy. See accountability in international humanitarian law and rule of law.

Controversies in CIHL often reflect broader disagreements about the proper balance between national security, humanitarian protection, and the political realities of international relations. From a pragmatic perspective, the enduring grievance is not the idea of limits on war but whether those limits are clear, consistently applied, and capable of withstanding the pressure of modern conflict. Advocates emphasize that a mature system of CIHL protects civilians, preserves the legitimacy of states, and reduces the likelihood of retaliation and long-term instability—outcomes that ultimately serve broad security interests.

See also