Prohibition on the Use of Force

The prohibition on the use of force is one of the most fundamental principles of contemporary Public International Law. It restricts States from using or threatening force against other States and forms a central part of the international legal system established after the Second World War.

The principle is primarily expressed in Article 2(4) of the United Nations Charter and is also recognised as a rule of customary international law. It protects State sovereignty, territorial integrity and political independence and seeks to replace unilateral military action with peaceful settlement and collective security.

The prohibition is not absolute in every circumstance. International law recognises specific circumstances in which force may lawfully be used, most importantly individual or collective self-defence following an armed attack and Security Council-authorised enforcement action.

Meaning of the Prohibition on the Use of Force

The prohibition means that States must refrain from the threat or use of force in their international relations where such force is prohibited by international law.

Article 2(4) of the UN Charter requires States to refrain from the threat or use of force against:

• the territorial integrity of another State;
• the political independence of another State; or
• in any other manner inconsistent with the purposes of the United Nations.

The principle therefore establishes a general prohibition against resorting to armed force as an instrument of international relations.

Historical Development

Before the twentieth century, international law did not contain a comprehensive prohibition on war.

States traditionally regarded war as a permissible instrument of national policy, subject to certain legal restrictions.

The legal position gradually changed through instruments such as:

• the Hague Conventions;
• the Covenant of the League of Nations;
• the Kellogg-Briand Pact of 1928.

The decisive development came with the establishment of the United Nations in 1945.

The UN Charter fundamentally changed the legal framework by establishing a general prohibition on the threat or use of force, while creating a system of collective security and recognising self-defence in specified circumstances.

Legal Basis

Article 2(4) of the UN Charter

Article 2(4) provides the principal treaty basis for the prohibition.

It requires UN Members to refrain from the threat or use of force against the territorial integrity or political independence of any State, or in any other manner inconsistent with the purposes of the United Nations.

The provision is central to the modern international legal order.

Customary International Law

The prohibition is not confined to the UN Charter.

The International Court of Justice recognised in Nicaragua v. United States that the principle concerning the prohibition of the use of force also exists under customary international law.

Consequently, the prohibition operates both:

• as a Charter obligation; and
• as a customary international law rule.

What Does “Force” Mean?

The principal understanding of “force” in Article 2(4) concerns armed or military force.

It includes conduct such as:

• military attacks;
• bombardment;
• invasion;
• military occupation resulting from unlawful force;
• attacks on another State’s territory;
• certain forms of military assistance;
• other acts involving prohibited armed force.

The precise classification of particular conduct depends upon its nature and circumstances.

Political and Economic Pressure

Not every form of pressure imposed by one State upon another constitutes “force” for the purposes of Article 2(4).

Economic or political pressure may raise separate questions under principles such as non-intervention, treaty law or other applicable rules.

Therefore, the concepts of:

• use of force;
• intervention;
• economic coercion;

should not automatically be treated as identical.

Threat of Force

Article 2(4) prohibits not only the actual use of force but also the threat of force.

A threat exists where a State indicates that it is prepared to use force in circumstances where such use would itself be unlawful.

The legality of a threat is therefore closely connected with the legality of the threatened use of force.

The International Court of Justice considered this relationship in the Legality of the Threat or Use of Nuclear Weapons Advisory Opinion.

The Court explained that a threat to use force would be unlawful if the contemplated use of force would itself be unlawful.

Scope of the Prohibition

The prohibition is directed primarily at force used in international relations.

It protects:

Territorial Integrity

States are protected against unlawful military attacks and territorial violations.

Political Independence

States must be free from unlawful military coercion aimed at controlling their political choices.

International Peace and Security

The prohibition supports the broader objective of preventing armed conflict and maintaining international peace.

Use of Force vs Intervention

The two concepts are closely related but distinct.

Point	Use of Force	Intervention
Main concern	Threat or use of armed force	Coercive interference in another State’s affairs
Primary rule	Article 2(4) and customary international law	Customary international law
Must armed force be involved?	Generally yes	Not necessarily
Example	Military attack on another State	Coercive support aimed at controlling another State’s political choices
Relationship	May also constitute intervention	May occur without armed force

A particular act may violate both principles.

Use of Force vs Aggression

The terms “use of force” and “aggression” are related but should not be treated as identical.

Use of force is the broader legal concept concerning prohibited resort to force.

Aggression refers to a particularly serious form of unlawful use of force and has developed as a specific concept in international law, including in relation to individual criminal responsibility under the Rome Statute framework.

Thus, every discussion of aggression involves force, but the legal analysis of every use of force does not necessarily amount to a determination of aggression.

Exceptions to the Prohibition

The prohibition on the use of force is fundamental, but international law recognises limited circumstances in which force may lawfully be used.

1. Individual Self-Defence

Article 51 of the UN Charter recognises the inherent right of individual self-defence if an armed attack occurs against a UN Member.

Self-defence therefore operates as an exception to the general prohibition.

The right is subject to the requirements imposed by international law.

2. Collective Self-Defence

A State may exercise collective self-defence where the legal requirements for collective self-defence are satisfied.

Collective self-defence involves assistance by one or more States to a State that has suffered an armed attack.

The Nicaragua v. United States case is particularly important for the conditions governing collective self-defence.

3. Security Council Authorisation

The UN Security Council may authorise measures involving the use of force under the collective security system established by Chapter VII of the UN Charter.

Such action is fundamentally different from unilateral military action because it derives its legal basis from the Charter’s collective security framework.

Self-Defence

Self-defence is the most important exception to the prohibition on force.

Article 51 recognises an inherent right of individual and collective self-defence when an armed attack occurs.

Requirements of Self-Defence

The exercise of self-defence is subject to the requirements of:

• necessity;
• proportionality;
• existence of an armed attack;
• compliance with applicable international law.

Necessity

Force used in self-defence must be necessary to respond to the armed attack.

Self-defence cannot be used simply as a convenient justification for military action unrelated to the attack.

Proportionality

The response must be proportionate to the armed attack and the legitimate defensive objective.

Proportionality does not necessarily mean that the defending State must use exactly the same amount of force as the attacker.

Rather, the response must remain within the limits required to address the armed attack.

Armed Attack

Article 51 specifically refers to an armed attack.

The International Court of Justice examined the concept in Nicaragua v. United States.

The Court distinguished between an armed attack and less serious forms of involvement.

It indicated that the sending of armed bands into another State’s territory may amount to an armed attack where the scale and effects are sufficiently serious.

However, the mere provision of weapons or logistical support to rebels does not automatically constitute an armed attack.

Collective Self-Defence

Collective self-defence involves assistance to a State that has suffered an armed attack.

The Nicaragua judgment identified important requirements for collective self-defence, including the need for the victim State to have been the subject of an armed attack and, in the circumstances considered by the Court, to have declared itself the victim and requested assistance.

Collective self-defence therefore cannot simply be invoked by a third State based solely upon its own assessment that another State has been attacked.

Anticipatory Self-Defence

The legality of anticipatory self-defence is a contested issue.

Article 51 refers to self-defence “if an armed attack occurs”.

Some States and scholars argue that customary international law may permit defensive action where an armed attack is imminent and waiting for the attack would make defence impossible.

Others adopt a stricter interpretation requiring an armed attack before force can lawfully be used in self-defence.

Therefore, anticipatory self-defence should not be presented as an entirely settled exception to Article 2(4).

Humanitarian Intervention

The use of force by one State or group of States for humanitarian purposes without Security Council authorisation remains legally controversial.

Some States and scholars have argued for the legality of unilateral humanitarian intervention in exceptional circumstances.

However, there is no generally accepted rule permitting unilateral humanitarian intervention as an undisputed exception to Article 2(4).

This issue must be distinguished from Security Council-authorised action and from the broader concept of the Responsibility to Protect.

Prohibition of Force and Non-Intervention

The two principles are closely connected.

Prohibition of Force

Prohibits the threat or use of armed force contrary to international law.

Non-Intervention

Prohibits unlawful coercive interference in matters within another State’s protected sphere.

Military intervention may violate both principles, but intervention can also occur without armed force.

Prohibition of Force and Sovereignty

The prohibition protects the sovereignty of States by preventing one State from using unlawful military force to control another State’s territory or political independence.

It therefore reinforces:

• sovereign equality;
• territorial integrity;
• political independence;
• non-intervention.

Important Cases

Nicaragua v. United States, 1986

The Military and Paramilitary Activities in and against Nicaragua case is one of the leading authorities on the prohibition of force.

The International Court of Justice held that the prohibition of the threat or use of force existed not only under the UN Charter but also as a rule of customary international law.

The Court found the United States responsible for violating the customary prohibition through certain acts involving Nicaragua.

The Court also rejected the United States’ justification of collective self-defence in the circumstances of the case.

The case remains fundamental for understanding:

• use of force;
• customary international law;
• self-defence;
• armed attack;
• collective self-defence;
• non-intervention.

Corfu Channel Case, 1949

The ICJ considered the sovereignty of States and rejected the idea that a State could unilaterally use force in another State’s territory merely to obtain evidence or protect its interests.

The case is important for the relationship between sovereignty and unilateral action.

Legality of the Threat or Use of Nuclear Weapons, 1996

The ICJ examined the legality of the threat or use of nuclear weapons under international law.

The Court confirmed the fundamental character of Article 2(4) and examined the relationship between the prohibition of force and the right of self-defence.

It also reaffirmed that necessity and proportionality are requirements governing self-defence.

Oil Platforms (Islamic Republic of Iran v. United States), 2003

The ICJ examined a claim of self-defence invoked by the United States in relation to attacks on oil platforms.

The case is important for the legal requirements governing self-defence, including the existence of an armed attack and the necessity and proportionality of the response.

Armed Activities on the Territory of the Congo, 2005

The ICJ examined the use of force and self-defence claims arising from military activities in the Democratic Republic of the Congo.

The judgment demonstrates the strict requirements applicable to reliance upon self-defence and reinforces the importance of territorial sovereignty.

Important International Instruments

United Nations Charter, 1945

The UN Charter provides the central framework for the prohibition of force.

Important provisions include:

• Article 2(4) — prohibition of the threat or use of force;
• Article 39 — Security Council determination of threats to peace, breaches of peace or acts of aggression;
• Article 42 — military enforcement measures by the Security Council;
• Article 51 — individual and collective self-defence.

Kellogg-Briand Pact, 1928

The General Treaty for Renunciation of War sought to renounce war as an instrument of national policy.

Although the international legal framework later developed substantially through the UN Charter, the Pact represents an important historical step toward the modern prohibition of force.

Friendly Relations Declaration, 1970

The Declaration reaffirmed the principle that States must refrain from the threat or use of force against the territorial integrity or political independence of other States.

It also connects the prohibition of force with sovereign equality, non-intervention and peaceful settlement.

Important Distinctions

Use of Force vs Self-Defence

Point	Use of Force	Self-Defence
General rule	Prohibited where contrary to Article 2(4) and applicable customary law	Recognised exception in specified circumstances
Legal basis	Article 2(4) and customary law	Article 51 and customary international law
Trigger	No lawful basis required for prohibited force	Armed attack
Requirements	Must comply with international law	Necessity and proportionality
Purpose	Varies	Defensive response

Self-Defence vs Reprisal

Point	Self-Defence	Armed Reprisal
Purpose	Respond to an armed attack	Punish or retaliate for a prior violation
Legal status	Recognised in specified circumstances	Armed reprisals are generally prohibited
Trigger	Armed attack	Prior wrongful conduct
Character	Defensive	Retaliatory

Security Council Action vs Unilateral Force

Point	Security Council Action	Unilateral Force
Basis	UN Charter collective security framework	Must have an independent legal justification
Authority	Security Council acting within its Charter powers	Individual State or States
Example	Chapter VII enforcement action	Unilateral military attack
Legal assessment	Depends on Charter authority and circumstances	Generally prohibited unless a recognised exception applies

Common Confusions

Is every use of military force unlawful?

Not necessarily.

The general rule prohibits unlawful force, but international law recognises specific circumstances such as self-defence following an armed attack and Security Council-authorised enforcement action.

Can a State use force simply because another State has violated international law?

Generally, no.

A prior violation does not automatically create a right to use armed force. International law provides other mechanisms for responding to wrongful conduct.

Is self-defence the same as retaliation?

No.

Self-defence is intended to respond to an armed attack and is subject to necessity and proportionality. Retaliatory armed force is not automatically lawful.

Does Article 2(4) prohibit only actual military attacks?

No.

It expressly prohibits both the threat and the use of force.

Is every economic sanction a use of force under Article 2(4)?

Not generally.

Economic measures may raise other questions under international law, including non-intervention and countermeasures, but they are not automatically treated as “force” for Article 2(4).

Can a State claim self-defence whenever it feels threatened?

No.

The right of self-defence is subject to legal requirements, including the armed-attack requirement under Article 51 and the principles of necessity and proportionality.

Is anticipatory self-defence universally accepted?

No.

Its legality remains contested, particularly regarding the relationship between Article 51’s reference to an armed attack and customary international law.

Is humanitarian intervention an undisputed exception to Article 2(4)?

No.

Unilateral humanitarian intervention without Security Council authorisation remains legally contested.

Topic at a Glance

Point	Position
Principle	Prohibition on the Threat or Use of Force
Primary legal basis	Article 2(4), UN Charter
Customary status	Also recognised under customary international law
Main objective	Protect peace, sovereignty, territorial integrity and political independence
General rule	States must refrain from prohibited threat or use of force
Main exception	Individual or collective self-defence following an armed attack
Collective security	Security Council may authorise enforcement action under Chapter VII
Key requirements of self-defence	Necessity and proportionality
Leading case	Nicaragua v. United States
Other important cases	Nuclear Weapons, Oil Platforms, Armed Activities
Contested issue	Anticipatory self-defence and unilateral humanitarian intervention

Quick Revision

• The prohibition on the use of force is a fundamental principle of modern Public International Law.
• Article 2(4) of the UN Charter is its principal treaty basis.
• The rule prohibits both the threat and use of force.
• The prohibition protects:
  • territorial integrity;
  • political independence;
  • international peace and security.
• The prohibition is also recognised as a rule of customary international law.
• The principal understanding of “force” under Article 2(4) concerns armed or military force.
• Political and economic pressure should not automatically be equated with armed force.
• The main recognised exceptions are:
  1. individual or collective self-defence following an armed attack;
  2. Security Council-authorised enforcement action.
• Self-defence is governed by necessity and proportionality.
• Article 51 recognises an inherent right of individual and collective self-defence.
• The existence of an armed attack is central to the Article 51 framework.
• Nicaragua v. United States is a leading case on the customary prohibition of force and self-defence.
• The Nuclear Weapons Advisory Opinion confirms the importance of necessity and proportionality in self-defence.
• Oil Platforms examines the requirements for relying upon self-defence.
• Armed reprisals are distinct from lawful self-defence.
• Anticipatory self-defence remains a contested issue.
• Unilateral humanitarian intervention without Security Council authorisation remains legally controversial.
• The prohibition of force and the principle of non-intervention are related but distinct.
• The prohibition of force is one of the foundations of the modern international legal order.

Conclusion

The prohibition on the threat or use of force represents one of the most significant developments in modern international law. By restricting unilateral resort to armed force, Article 2(4) of the UN Charter transformed the legal framework governing international relations and placed peaceful settlement and collective security at its centre. The rule is reinforced by customary international law and supported by principles of sovereignty, territorial integrity and political independence. Although international law recognises limited circumstances in which force may lawfully be used, particularly self-defence and Security Council-authorised enforcement, these exceptions are subject to specific legal requirements. The prohibition therefore remains a central safeguard against the unilateral use of military power in international relations.