Use of Force, War, and the Principles of Neutrality in International Relations

Index of Headings

1.Concept of War
2.Laws of War (International Humanitarian Law)

• 2.1 Declaration of War
• 2.2 Effects of War
• 2.3 Enemy Character 

3.Prisoners of War (POWs) 
4.Neutrality

• 4.1 Meaning
• 4.2 Rights and Duties of Neutral States
• 4.3 Neutral Territory 

5.Prohibition of Use of Force

• 5.1 UN Charter Framework
• 5.2 Self-Defence 

6.Modern Concept of Armed Conflict 
7.Conclusion 
8.FAQs

1.Concept of War

War, when used as an expression within public international law, has traditionally been understood as a legal condition involving armed hostilities (armed conflict) between two or more legitimate nation states. In terms of its definition; however, war is not merely the existence of violence per se and/or the creation of a factual existence of violence between the parties but rather, the creation of a legal entity with specific rights and obligations attached to the parties involved. The substance/definition of war is defined by Oppenheim to be 'A contention (conflict) between two or more State powers, by the use of their armed forces, for the purpose of overpowering each other and imposing on their subordinates such terms as the victor pleases in respect of peace'.

Prior to the 20th Century, war (in a political sense) was viewed as a legitimate means to reach state policy goals and was considered a right inherently connected to State sovereignty. The belief that nations should be able to go to war in pursuit of their policy goals is reflected in the concept of bellum justum (just war) that was developed by many international law theorists, i.e. Grotius and Vattel and later by Vitoria as well. A 'just war' was defined as a war with a viable justifiable cause (i.e. self-defence or compensation for an injury), right intention, proportionality, and the conduct of the war as a last resort.

The first and second world wars marked the dawning of a new era in modern warfare and state violence and highlighted the horrific consequences created by a state’s right to engage in war without limitations. Furthermore, the establishment of international law's prohibition of war as an instrument of national policy can be traced back to the restrictions imposed on the use of force in the League of Nations Covenant (1919). These restrictions were further enhanced through the Kellogg-Briand Pact (1928), which sought to completely eliminate the ability of states to utilise war as an instrument of national policy.

2. Laws of War

The laws of war also known as International Humanitarian Law (IHL) or the Law of Armed Conflict (LOAC) constitute a set of rules that seek to limit the effects of armed conflict. They protect persons who are not or are no longer participating in hostilities, and restrict the means and methods of warfare. The principal sources are the Hague Conventions (1899, 1907), the Geneva Conventions (1949), and their Additional Protocols (1977, 2005).

2.1 Declaration of War
Historically, a formal declaration was required to commence war. A declaration of war (or ultimatum with conditional declaration) would give notice before official hostilities began. Article 1 of the Hague Convention III (1907) made it official by stating that hostilities cannot start without notice in the form of declaration of war or ultimatum with conditional declaration.

However, when World War II began, many countries did not have a formal declaration to use force against other countries; Japan’s attack on Pearl Harbour in December 1941 was an example of this type of situation. Presently, most conflicts are armed conflicts that begin without having formal declarations of war. In addition, if there were a formal declaration of war (under UN Charter), it would no longer have legal significance because a war in and of itself is prohibited by the Charter. Modern law applies to any situation where an armed conflict exists and is based on factual conditions regardless of whether there was a formal declaration of war.

In the S.S. Lotus Case (PCIJ, 1927), the Court decided that international law will not restrain states from using force against other states, except to the extent that international law expressly forbids states from doing so. This case serves as the point of reference regarding the pre-Charter authority given to states allowing them to engage in war.

2.2 Effects of War
The formal commencement of war between states produces significant legal consequences:
First, diplomatic and consular relations between belligerent states are severed. Second, treaties in force between the belligerents may be suspended or terminated, although obligations under multilateral humanitarian treaties persist. Third, enemy nationals on a state's territory may be subjected to special restrictions. Fourth, enemy property, both public and private become subject to capture and confiscation according to the law of prize. Fifth, commercial relations between nationals of belligerent states are suspended.

The laws of war apply in full once a state of international armed conflict is established, regardless of whether the conflict is declared or undeclared, and regardless of the justness of the cause of either party (the principle of equal application of IHL).
— Geneva Convention IV (1949), Article 6: Provides for the application of the Convention from the outset of any conflict referred to in Article 2.

2.3 Enemy Character
Enemy character is a concept by which individuals and entities are classified as belonging to the enemy and are therefore subject to certain disabilities or liabilities under the laws of war. The test for enemy character has traditionally been based on nationality, though domicile and business connections have also been used.

In the case of Daimler Co. v Continental Tyre & Rubber Co. (1916), through a public court in England known as the House of Lords and by applying the control test, they determined that this particular company was nevertheless considered an enemy because at the time of war it had predominantly and perhaps exclusively, German shareholders and directors making this company also German owned at the present time

Additionally, according to Article 4(1) of the Geneva Convention III (1949), anyone who is part of the Armed Forces of the enemy must be held as a POW; if they are not members of the Armed Forces then they must meet other requirements to hold POW status Article 4(2-3) outlines specifically the requirements for:

(1) Members of the armed forces of a Party to the conflict
(2) Members of militias or volunteer corps belonging to a Party to the conflict;
(3) Members of organized resistance movements; provided that the persons meet certain restrictions outlined in Article 4(2-3) & must meet all necessary conditions.

3. Prisoners of War

The assurance of the rights of prisoners of war (POWs) is among the greatest accomplishments in International Humanitarian Law (IHL). A prisoner of war is defined as a member of an opposing army who has been captured. The Third Geneva Convention (1949) establishes a complete legal framework for the treatment of POWs.
Under Article 4 Geneva Convention III provides for the granting of POW status to:
1. Members of the armed forces of one of the parties to a conflict
2. Members of militia and volunteer corps that are part of that country's armed forces
3. Members of unrecognized militia or volunteer corps including resistance movements that are part of one of the parties to the conflict providing these groups:
- are directly commanded by someone with legal or other authority
- distinguish themselves from the civilians they are operating with; and
- carry their arms openly and comply with the laws of war.
4. The civilian population who has been accompanying the armed forces and is separately defined as being part of those forces by virtue of having the right to carry arms.

— Geneva Convention III (1949), Article 17: A POW is bound to give only name, rank, date of birth, and service number upon interrogation. They cannot be compelled to provide any other information.
— Geneva Convention III (1949), Article 118: POWs shall be released and repatriated without delay after the cessation of active hostilities.

POWs must at all times be humanely treated. They must be protected against acts of violence or intimidation and against insults and public curiosity (Article 13). Reprisals against POWs are prohibited (Article 13). They retain their legal personality and may not be compelled to take part in military operations against their own country. The detaining power bears responsibility for their welfare.

The question of POW status has arisen sharply in the context of modern asymmetric warfare — particularly regarding alleged terrorists captured by the United States after 9/11. In Hamdan v Rumsfeld (2006), the US Supreme Court held that Common Article 3 of the Geneva Conventions applies to the conflict with Al Qaeda, rejecting the US Government's position that the Conventions did not apply at all to such detainees.

Hamdan v Rumsfeld 548 U.S. 557 (2006), US Supreme Court: Held that Common Article 3 of the Geneva Conventions applies to detainees in the conflict with Al Qaeda, establishing minimum standards of treatment even in non-international armed conflicts.

4. Neutrality

4.1 Meaning
A neutral state refers to a State which has decided not to actively and militarily participate in an ongoing armed conflict between other States. The State will maintain an impartial position (neutral) with respect to the states that are engaged in warfare. For example, a neutral power will have the right to have its citizens travel to and through areas of conflict and, subject to compliance with international law, to trade with belligerent nations. Law relating to neutrality is found in the Hague Conventions of 1907: Hague Convention V (regarding the rights and responsibilities of neutral nations and their citizens in land warfare) and Hague Convention XIII (regarding the rights and responsibilities of neutral nations with regard to naval warfare).

The term non-belligerency is a distinct concept from neutrality. A State that is non-belligerent is not participating in a war; however, it might assist in some capacity one of the parties to the armed conflict. A State which has established its status of neutrality by treaty or through its Constitution is referred to as having permanent neutrality (e.g. Switzerland).

4.2 Rights and Duties of Neutral States
States that are neutral gain specific rights and take on related obligations. Their main right is inviolability, which prohibits belligerents from attacking, recruiting, or passing through neutral territory. Subject to the laws of contraband and blockade, neutral states are also permitted to maintain commercial relations with all belligerent parties. 

One of the responsibilities of neutral states is impartiality: a neutral state cannot support any belligerent directly militarily or take sides with them. It must stop belligerents from using its territory as a base of operations. A neutral state may, however, allow private traders to export illegal goods to belligerents and offer non-military humanitarian aid to all sides equally.

— Hague Convention V (1907), Article 1 — The territory of neutral powers is inviolable.
— Hague Convention V (1907), Article 5 — Neutral powers must not allow the belligerents to use their territory for the movement of troops or convoys of supplies.

The 1956 Alabama Claims Arbitration between the United States and Great Britain remains a landmark in the development of neutral duties. Britain was held liable for allowing Confederate warships (including the CSS Alabama) to be built in British ports and used against the Union. The arbitral tribunal awarded the US $15.5 million, establishing the principle that a neutral state has a duty to prevent its territory from being used to equip vessels for use by a belligerent.

Alabama Claims Arbitration (1872), US v United Kingdom: Established that a neutral state incurs international responsibility if it fails to prevent its territory from being used by belligerents; foundational for the law of neutral duties.

4.3 Neutral Territory
Neutrality's fundamental rule is that armed forces cannot pass through a neutral nation or set up bases or conduct military operations from it. Thus, there is no way for neutral nations to allow belligerent states to fly military aircraft above their territory without violating their neutrality.

A neutral state has the right (and duty) to use force to repel any intrusion into its own territory by a belligerent state. When belligerents enter into the territory of a neutral state, they must be disarmed and kept as prisoners for the duration of the conflict. Wounded members of the military can be treated, but must be prevented from rejoining their forces.

The ICJ confirmed the principle of the Corfu Channel Case (1949) and agreed with the principle of a state being unable to use the territory of another state for acts that violate the rights of other states, which is an affirmation of the inviolability of a nation's territory during both peace and war.
Corfu Channel Case (UK v Albania), ICJ Reports 1949 — Held that Albania was responsible for the consequences of mines laid in its territorial waters; affirmed the principle of territorial sovereignty and a state's duty not to allow its territory to be used for acts harmful to other states.

5. Prohibition of Use of Force

5.1 UN Charter
The fundamental tenet of contemporary international law that governs relations between states is the ban on the use of force. "In their international relations, all Members shall 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," states Article 2(4) of the UN Charter.

— UN Charter, Article 2(4): Prohibits the threat or use of force against the territorial integrity or political independence of any state.
— UN Charter, Article 39: Authorizes the Security Council to determine the existence of any threat to peace, breach of peace, or act of aggression, and to recommend or decide measures.
— UN Charter, Article 42: The Security Council may take military action as may be necessary to maintain or restore international peace and security.

Article 2(4)'s prohibition is regarded as a jus cogens norm, which is a preemptive standard of general international law that cannot be deviated from. It includes threats of force in addition to actual use of force. The scope of this prohibition has been further clarified by UN General Assembly Resolution 2625 (1970), which is the Declaration on Principles of International Law; Resolution 3314 (1974); and the Definition of Aggression. 

In the Nicaragua Case (1986), the International Court of Justice (ICJ) upheld the prohibition's status as customary international law, ruling that the United States had violated the UN Charter's ban on the use of force by aiding the Contra rebels in Nicaragua.

Case Concerning Military and Paramilitary Activities in and against Nicaragua (Nicaragua v USA), ICJ Reports 1986, p. 14 the ICJ held that both treaty and customary international law prohibit the use of force; the US violated international law by mining Nicaraguan harbours and supporting the Contras. Reaffirmed non-intervention and prohibition on force as jus cogens.

5.2 Self-Defence
Article 51 of the UN Charter preserves the 'inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security.'
— UN Charter, Article 51:  Preserves the inherent right of individual or collective self-defence against an armed attack, pending Security Council action.
Both treaty law and customary international law recognize the necessity and proportionality requirements that apply to the right to self-defense. The traditional source of these standards is the Caroline Affair (1837). US Secretary of State Webster developed the requirements after British forces destroyed the ship Caroline on US soil: the response must not be "unreasonable or excessive," and the need for self-defense must be "instant, overwhelming, leaving no choice of means, and no moment of deliberation."

The Caroline Affair (1837), US-UK Diplomatic Exchange: Formulated the classical criteria for self-defence: the threat must be instant, overwhelming, leaving no alternative, and the response must be proportionate. These criteria remain foundational in customary international law.

The ICJ has addressed self-defence in several landmark cases. In the Nicaragua Case, the Court held that for collective self-defence to be lawful, the victim state must declare itself the object of an armed attack and request assistance. In Oil Platforms (Iran v USA, 2003), the Court emphasised that the attacking state must demonstrate that the actions constituting the armed attack were indeed attributable to the defending state.

A debated right under Article 51 whether anticipatory or pre-emptive self-defence exists; that is, the ability to use force against an imminent yet to occur armed attack. The Caroline Test outlines the possibility of anticipatory self-defence when there is imminent threat; otherwise, a restrictive interpretation will need the existence of an actual armed attack. The US National Security Strategy (2002) controversially extends this idea of ‘pre-emptive’ action to include taking action on an imminent threat from a distant source. The use of pre-emptive action, also known as the ‘Bush Doctrine’, has not achieved a level of general acceptance in International Law.

Oil Platforms Case (Iran v USA), 2003 ICJ Reports: The ICJ required that evidence of an alleged attack must first be established before relying on self-defence as a defence for military action by the US; reinforced the principles of necessity and proportionality as requirements.
Armed Activities on the Territory of the Congo (DRC v Uganda), 2005 ICJ Reports: Held that Uganda was responsible for its use of force in the DRC; further clarified that extraterritorial self-defence may only be triggered by an attack on the host state by a non-state actor if that attack is attributable to that state or the host state is unable/unwilling to prevent any such attack.

6. Modern Concept of Armed Conflict

The broader legal category of "armed conflict," which includes a wider range of hostilities, has replaced the traditional binary classification of war/peace. International armed conflict (IAC) and non-international armed conflict (NIAC) are the two main categories that modern IHL differentiates. 
Every time two or more states use force, there is an international armed conflict that is governed by all four Geneva Conventions and Additional Protocol I (1977) (Common Article 2). According to Article 1(4) of Additional Protocol I, it also covers wars of national liberation. Common Article 3 and Additional Protocol II (1977) define non-international armed conflict as hostilities between a state and non-state armed groups or between such groups on a state's territory.
— Additional Protocol I (1977), Article 1(4):  Extends the application of IAC rules to armed conflicts in which peoples are fighting against colonial domination, alien occupation, or racist regimes.
— Additional Protocol II (1977), Article 1:  Applies to armed conflicts between armed forces of a state and dissident armed forces or other organised armed groups, exercising territorial control.
The ICTY in Prosecutor v Tadić (1995) established the foundational definition of armed conflict: 'an armed conflict exists whenever there is a resort to armed force between States or protracted armed violence between governmental authorities and organized armed groups or between such groups within a State.' The Tadić criteria, intensity of violence and organisation of the armed group remain the standard test for NIAC.
Prosecutor v Tadić (IT-94-1), ICTY Appeals Chamber, 1995:  Laid down the threshold criteria for armed conflict i.e. intensity and organisation. Held that Common Article 3 reflects customary international law applicable in all armed conflicts.

The broader legal category of "armed conflict," which includes a wider range of hostilities, has replaced the traditional binary classification of war/peace. International armed conflict (IAC) and non-international armed conflict (NIAC) are the two main categories that modern IHL differentiates. 
Every time two or more states use force, there is an international armed conflict that is governed by all four Geneva Conventions and Additional Protocol I (1977) (Common Article 2). According to Article 1(4) of Additional Protocol I, it also covers wars of national liberation. Common Article 3 and Additional Protocol II (1977) define non-international armed conflict as hostilities between a state and non-state armed groups or between such groups on a state's territory.

7. Conclusion

The law governing the use of force, war, and neutrality represents one of the most dynamic and consequential areas of international law. From the classical laws of war rooted in natural law philosophy, through the codification of the Geneva and Hague Conventions, to the revolutionary prohibition of the use of force under the UN Charter, international law has undergone a fundamental transformation in its approach to armed conflict.

The concept of war has evolved from a permissible instrument of state policy to a prohibited act under jus cogens. The laws of war through the Geneva Conventions and Additional Protocols have progressively expanded the protection of civilians, prisoners of war, and the wounded. The law of neutrality, though under strain in an era of collective security, continues to provide important protections for states that choose to remain outside armed conflicts.

The UN Charter's prohibition of the use of force, with its limited exception for self-defence under Article 51 and collective enforcement under Chapter VII, constitutes the cornerstone of the contemporary international legal order. The ICJ's jurisprudence from the Nicaragua Case to Oil Platforms and the Armed Activities case has progressively refined the conditions under which force may lawfully be used.

Yet significant challenges remain. The rise of non-state actors, the phenomenon of transnational terrorism, hybrid warfare, and cyber operations all test the limits of existing legal frameworks. The modern concept of armed conflict, while expanded by the ICTY's Tadić decision, still struggles to accommodate new forms of violence. International law must continue to adapt to these realities while preserving its foundational principles of humanity, necessity, proportionality, and distinction principles which constitute the bedrock of a rules-based international order.

FAQs

1. What is the difference between ‘war’ and ‘armed conflict’ in modern international law?
Traditionally, ‘war’ referred to a formally declared legal state between sovereign states. Modern international law prefers ‘armed conflict,’ which focuses on the factual existence of hostilities rather than formal declarations. This shift ensures that humanitarian rules apply regardless of whether war is officially declared. It broadens protection under international humanitarian law.

2. When is the use of force allowed under the UN Charter?
The UN Charter generally prohibits the use of force under Article 2(4). However, exceptions exist: self-defence under Article 51 and actions authorized by the UN Security Council under Chapter VII. Self-defence must meet the requirements of necessity and proportionality. These limits aim to prevent abuse while allowing states to respond to genuine threats.