Introduction 

Nuclear weapons occupy a unique position in international law due to their unprecedented destructive capacity, with legal restrictions deriving from multiple sources of international law including jus ad bellum (law governing interstate use of force) and jus in bello (international humanitarian law applicable in armed conflict). 

Nuclear Weapons Under International Humanitarian Law (Jus in Bello) 

Fundamental Principles and Rules 

International Humanitarian Law (IHL) imposes several fundamental rules that significantly constrain any potential use of nuclear weapons: 

• Rule of Distinction:  
  • Parties to a conflict must direct attacks only against lawful military objectives.  
  • Any weapon incapable of distinguishing between civilians/civilian objects and military targets is considered inherently indiscriminate, rendering its use unlawful.  
  • This rule is part of customary international law. 
• Rule of Proportionality:  
  • Even when directed at military objectives, attacks must not cause civilian harm (deaths, injuries, damage to civilian objects) that would be excessive compared to the direct and concrete military advantage anticipated. 
• Prohibition of Unnecessary Suffering:  
  • IHL prohibits the use of means and methods of warfare that cause superfluous injury or unnecessary suffering to combatants. 

Specific Characteristics of Nuclear Weapons Relevant to IHL 

Nuclear weapons create unique and severe effects that raise serious IHL concerns: 

• Heat Effects:  
  • Detonation creates temperatures between 60-100 million degrees centigrade.  
  • Unprotected individuals within 2.5km of ground zero will suffer fatal third-degree burns. 
• Radiation Effects:  
  • Nuclear weapons produce both "prompt" radiation (neutrons, gamma rays, electrons) immediately after explosion and delayed radioactive fallout beginning one to two hours afterwards. These effects cause:  
    • Immediate lethal radiation exposure 
    • Long-term cancer mortality risks that persist throughout survivors' lives 
• Temporal Dimension:  
  • The injuries and suffering from radiation exposure extend long beyond the immediate attack, raising specific concerns under the unnecessary suffering rule. 

Scenarios of Potential Use and Their Legal Assessment 

• Use Against Discrete Military Targets: In highly specific scenarios (e.g., a nuclear depth-charge against a submarine about to launch nuclear missiles in a remote area), the use might potentially satisfy IHL requirements, provided:  
  • The target location is precisely known 
  • Requisite authority has been secured 
  • Proportionality calculations confirm no excessive civilian harm 
  • No less harmful alternatives would achieve the military objective 
• Use Against Enemy Forces in Remote Areas: Use against enemy combatants in isolated areas (e.g., a desert) might theoretically meet distinction and proportionality tests in certain circumstances. However, such use would likely violate the prohibition on unnecessary suffering due to radiation effects. 
• Use as Belligerent Reprisal: For a reprisal to be lawful, it must:  
  • Be a necessary response to a prior serious violation of IHL 
  • Be conducted to bring the violating state back into compliance with IHL 
  • Be announced as such 
  • Be proportionate to the original breach 

However, numerous categories of persons and objects enjoy special protection against reprisals under IHL, including civilians. While some states (e.g., UK) have attached understandings to Additional Protocol I reserving the right to reprisals against civilians in extreme circumstances, a massive nuclear response to a first strike would likely constitute unlawful retaliation rather than a lawful reprisal. 

ICJ Advisory Opinion 

• In its 1996 Advisory Opinion on the Legality of the Threat or Use of Nuclear Weapons, the International Court of Justice concluded that any use of nuclear weapons would "generally be contrary to the rules of international law applicable in armed conflict, and in particular the principles and rules of humanitarian law." 
• The Court acknowledged that truly exceptional circumstances might exist, particularly regarding low-yield nuclear weapons, where use might satisfy legal requirements.  
• However, it remains implausible to argue that all potential uses of nuclear weapons would be inherently indiscriminate or disproportionate under IHL. 

Nuclear Weapons Under International Criminal Law 

Potential Criminal Liability 

The use of nuclear weapons could, depending on circumstances and liability modes, constitute: 

• Genocide: If committed with intent to destroy, in whole or part, a national, ethnical, racial, or religious group 
• Crimes Against Humanity: If undertaken as part of a widespread or systematic attack against a civilian population where the perpetrator has knowledge of the attack 
• War Crimes: Through violations of IHL rules 

Jurisdiction Issues 

While the Rome Statute of the International Criminal Court (ICC) lacks explicit jurisdiction regarding nuclear weapon use, this does not preclude categorization of such use as an international crime under other legal regimes or subject to national prosecution. 

Human Rights Law Considerations 

International human rights law is relevant to determining the legality of nuclear weapons use, particularly regarding the right to life. Human rights courts analyze whether sufficient effort was made to avoid or limit loss of life in cases where potentially lethal force cannot be avoided. 

Unlike IHL's proportionality analysis, human rights law does not balance military advantage against civilian casualties. The positive obligations required under human rights law mean that any use of nuclear weapons would likely result in concrete human rights violations that are justiciable where the responsible state has jurisdiction. 

Non-State Actors and Nuclear Weapons 

Terrorism Risk 

The use of nuclear weapons by non-state actors as acts of terrorism represents a significant concern. Intelligence assessments have indicated that various terrorist groups have pursued or expressed interest in obtaining nuclear capabilities. 

Legal Framework 

The treaty regime prohibiting armed non-state actors' access to nuclear weapons and material is fragmented and often overlapping. UN Security Council Resolution 1540 (2004) obligates all states to: 

• Refrain from providing support to non-state actors attempting to develop, acquire, manufacture, possess, transport, transfer, or use nuclear weapons 
• Adopt and enforce appropriate laws prohibiting such activities 
• Establish domestic controls to prevent proliferation 

Jus ad Bellum Considerations 

Use of Force Framework 

Under jus ad bellum (law governing interstate use of force), nuclear weapons could potentially be used in self-defense against an armed attack if the requirements of necessity and proportionality are satisfied: 

• Necessity: Requires that there be no reasonable alternative to using force. This involves a contemporaneous and bona fide belief that force is necessary. 
• Proportionality: Force used in self-defense should:  
  • Be assessed in light of fulfilling defensive purposes 
  • Not be obviously excessive (though it need not be strictly proportionate to the offensive force) 

ICJ Advisory Opinion on Self-Defense 

The ICJ found that it "cannot conclude definitively whether use of nuclear weapons would be lawful or unlawful in an extreme circumstance of self-defense, in which the very survival of a State would be at stake." 

Separation of Jus ad Bellum and Jus in Bello 

• A critical legal principle is that jus ad bellum and jus in bello operate independently. 
• The legitimacy of a state's reason for going to war (jus ad bellum) does not affect the application of humanitarian rules during that conflict (jus in bello). 
• The "conflationist" position, which seeks to subordinate jus in bello to jus ad bellum, contradicts established treaty and customary international law.  
• IHL would disintegrate if its application depended on the perceived lawfulness of the initial use of force. 
• Therefore, the use of nuclear weapons in a manner that violates IHL cannot be considered consistent with international law, regardless of the justification under jus ad bellum. 

Conclusion 

The legal status of nuclear weapons under international law is governed by multiple, complementary legal regimes. While no comprehensive prohibition of nuclear weapons exists in customary international law, their use is significantly constrained by fundamental rules of IHL, human rights law, and jus ad bellum.