Illustration: Elena Scotti (GMG)

This piece is part of a recurring series that aims to be a complete guide to the laws of war. You can read previous entries here.


Civilians bear many of the negative effects of armed conflict, and women and children are disproportionately affected. Rape and sexual violence are perpetrated on a large scale in times of conflict. Areas affected by conflict see higher levels of poverty and malnutrition, and lack of access to children’s education, food, and water. Situations of violence create huge numbers of refugees and internally displaced persons, destroy critical infrastructure that benefit society, seriously harm the natural environment, and ruin entire economies. The effects of violence breed future violence.

For all these reasons, the prohibition on the use of armed force is the single most important rule in all of international law. The Charter of the United Nations prohibits the threat or use of force by any state against the territorial integrity or political independence of any other, with only two prescribed exceptions.

The Use of Force

Through thousands of years of humankind’s existence, there were no real limits on rulers’ ability to wage war—initiating armed conflict was a matter of policy, not of law. Though various notions of “just war” existed in teachings in many civilizations over centuries, until relatively recently invasions were seen as legitimate methods of annexing territory, punishing enemies, or enforcing contracts. Sun Tzu wrote that, “The art of war is of vital importance to the State. It is a matter of life and death, a road either to safety or to ruin.”


After World War I, the League of Nations was created with the intention that the peoples of the world would pursue a peaceful and secure world. The League’s Covenant wagged its finger at the idea of acts of aggression without banning them outright; its preamble referred to “obligations not to resort to war,” but its operative provisions only required assurances that members “undertake to respect and preserve against external aggression,” and indeed only required states to wait three whole months after a dispute had been through arbitration before resorting to war.

The League famously failed in its achieving its ideals, hamstrung by the absence of world powers in its leadership (the United States refused to join; the Soviet Union did not join for more than a decade and was subsequently expelled for invading Finland) and finding itself unable to prevent acts of aggression throughout the 1930s and 1940s. The United Nations was formed in the ashes of World War II, and its Charter historically proclaimed some of the cornerstone principles of modern international law: independence and sovereign equality of states, equal rights and self-determination of peoples, non-interference in internal affairs of states, respect for human rights, friendly relations among states, and, crucially, the prohibition on the use of force. These days, the UN enjoys universal membership of the nations of the world.

The United Nations Charter prohibits the use of force by a state against another, permitting exceptions only in two circumstances set out in the Charter. Article 2(4) reads:

All Members shall refrain in their international relations 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.


As we’ll see below, the Charter authorizes the use of force if employed in individual or collective self-defense, or when it has been approved by the United Nations Security Council. Force may therefore not be used for any other reason—in revenge, as a countermeasure, or to belligerently occupy or annex territory.

A state may also exercise its sovereign right to give consent to another state to use force on its territory. The Articles on Responsibility of States for Internationally Wrongful Acts provide that valid consent of a state to the act of another is a circumstance precluding the wrongfulness of that act. (Some regard the use of force with consent as a third exception to the prohibition, while others view it as not inconsistent with the rule—as it would not be a use of force “against” the territorial integrity or political independence of the state—and therefore not an exception per se. It doesn’t matter which view you take, though here I’m not counting it as an exception.)

The UN Charter is bolstered by the famous Article 103, the most consequential in all of international law, which provides that Charter obligations prevail in the event of a conflict with obligations under any other international agreement. In other words, states cannot “contract out” of its obligations under the Charter.



The first exception to the prohibition—and by far the most commonly relied upon—is the recognition of states’ inherent right to use force in individual and collective self-defense. Everyone agrees that the right to self-defense is an indispensable pillar of the law on the use of force, but there is significant disagreement as to the precise contours of the right.

Article 51 of the UN Charter provides (in part):

Nothing in the present Charter shall impair 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.


The word “inherent” in Article 51 indicates (should it have ever been in doubt) that the right of self-defense exists at customary international law—an unwritten body of law which develops in recognition of states’ practice and their acceptance of the practice as law—as well as pursuant to treaty law under the UN Charter. The International Court of Justice has stated that the areas governed by the law of self-defense under the Charter and at customary law “do not overlap exactly, and the rules do not have the same content.”

A state which acts in self-defense under Article 51 must report its action to the United Nations Security Council. This is a procedural requirement rather than a substantive one—that is, a failure to comply with this condition cannot turn a lawful exercise of the right of self-defense into an unlawful one. Nevertheless, the International Court of Justice stated in 1986 that the absence of a notification may indicate for the purposes of customary law that a state was not itself convinced that it was acting in self-defense, and since then states have far more rigorously complied with the reporting requirement.

It is uncontroversial that measures of self-defense (whether individual or collective) must be necessary (in that no alternative response is possible) and proportionate (relating to the size, target, and duration of the response).


There can be considerable difficulty in defining exactly what types of actions can constitute an “armed attack” for the purposes of triggering the right to self-defense under Article 51. An armed invasion by state forces obviously reaches the threshold, but tricky questions are raised by non-kinetic cyber operations, acts of violence by irregular forces, and isolated incidents of unclear intent and lesser gravity. There is a split between developed and developing states as to whether economic coercion can amount to a use of force.

States may act in collective self-defense of its allies even when its individual right to use force in self-defense would not be triggered. Article 5 of the NATO treaty provides that its members agree that an armed attack against any one of them shall be considered an attack against each of them enlivening their right (and intent) to use force in collective self-defense. This provision has only needed to be relied upon once in its near-70 years, after the attacks of 9/11.


9/11 brought the issue of self-defense against non-state armed groups in response to terrorism to the forefront of the international arena. In the war on terrorism the United States championed the Bush doctrine of anticipatory self-defense: that force may be used as a purely pre-emptive action, in the absence of any imminent threat. The Bush doctrine risked stretching the right to self-defense to its breaking point, blurring the lines between genuine self-defense and unilateral acts of aggression, and received very little support in the international arena, being repeatedly rejected by the hundred-plus nations of Non-Aligned Movement and conspicuously not supported by NATO, the European Union, or the United Kingdom.

The recent coalition actions against ISIL in the Middle East featured overlapping legal justifications. Those fighting against ISIL on Iraqi territory were, from September 2014, doing so with the express consent and at the request of the government of Iraq, while at least ten nations wrote to the Security Council under Article 51 to justify their use of force against ISIL on Syrian territory, largely (see e.g. the United States) on the basis of collective self-defense of its ally Iraq.

United Nations Security Council Approval

The second exception to the prohibition is the permission to use force with the United Nations Security Council’s approval. The Security Council is authorized under Chapter VII of the UN Charter to determine the existence of any threat to the peace, breach of the peace, or act of aggression, and to authorize the use of force to maintain or restore international peace and security.


Article 42 of the UN Charter provides:

Should the Security Council consider that measures provided for in Article 41 [measures not involving the use of force] would be inadequate or have proved to be inadequate, it may take such action by air, sea, or land forces as may be necessary to maintain or restore international peace and security. Such action may include demonstrations, blockade, and other operations by air, sea, or land forces of Members of the United Nations.

The Security Council consists of 15 member states, being 10 rotating members plus the permanent five—China, France, Russia (which inherited its seat from the USSR), the United Kingdom, and the United States. On substantive matters, decisions of the UNSC must be passed by nine members, including “the concurring votes” of the P5, the famous veto power. (A resolution may pass if one or more of the P5 abstain, but not if they cast a negative vote.)


It scarcely needs to be said that the Security Council is nearly always politically deadlocked. The specific combination of the nations occupying the five permanent seats is a certain recipe for disagreement on almost any possibly conceivable concern, let alone on important global security issues. The membership of the rotating seats often doesn’t aid matters—morbidly, Rwanda began a two-year rotation on the Security Council at the start of 1994, and its ambassador used the seat to argue against intervention in its civil war, and fed his inside knowledge of the Council’s deliberations to the Hutu extremists who used that information to their advantage in planning and executing the genocide of more than one million Tutsis.

The Security Council doesn’t have its own armed forces—Article 43 of the Charter envisages that states would make troops available to the Security Council on call, but that has never materialized—and so in authorizing the use of force it must therefore delegate to member states.

The political paralysis brought about by the Cold War meant that the Security Council only authorized the use of force once from the birth of the United Nations in 1945 until 1990 (in Korea in 1950). The collapse of the Soviet Union and the end of the Cold War gave rise to hopes for a new era of cooperation in the UN as it headed toward the 21st century. After Iraq invaded Kuwait in August 1990, the Security Council awoke from its decades-long slumber, and in the famous Resolution 678 authorized member states to use “all necessary means” to secure the withdrawal of Iraqi forces.


Acting under its Chapter VII mandate, the Security Council has since then used that magic euphemistic phrase (or sometimes “all necessary measures”) to authorize the use of force in various instances including in relation to Somalia (to establish “a secure environment for humanitarian relief operations”), Haiti (“to facilitate the departure from Haiti of the military leadership”), Libya (“to protect civilians and civilian populated areas under threat of attack”), Timor-Leste (“to restore peace and security … [and] to facilitate humanitarian assistance operations”), Chad and the Central African Republic (“to contribute to protecting civilians in danger [and] to facilitate the delivery of humanitarian aid”), and Mali (“in support of transitional authorities and to bring to justice those responsible for war crimes and crimes against humanity”), amongst other examples.

The Security Council famously authorized the use of force to address terrorism in response to the attacks of 9/11, unanimously passing a wide-ranging resolution that variously required states to prevent, criminalize, and freeze funds involved in the financing of terrorist acts, demanded that they act early to prevent terrorism and prevent the movement of terrorists, and called on states to work together to suppress terrorism. The Security Council expressed its determination to “take all necessary steps in order to ensure the full implementation” of the resolution.

In 2003, coalition actions in Iraq again marked a new moment in the practice of the Security Council. The United States and United Kingdom had lobbied the Security Council to explicitly authorize the use of force against Saddam Hussein’s regime, without success. Undeterred, a coalition led by those nations initiated “Operation Iraqi Freedom” to invade Iraq. The U.S. and U.K. offered a complex legal justification relying on an implied Security Council authorization to use force based on a combination of: (i) the 1990 resolution referred to above which authorized force against Iraq in response to its invasion of Kuwait; (ii) a 1991 resolution which imposed a ceasefire and required Iraq to eliminate and refrain from acquiring nuclear weapons; and (iii) a 2002 resolution which found Iraq in material breach of those disarmament obligations.


The Non-Aligned Movement and the League of Arab States each wrote to the UN to call the use of force an illegal act of aggression in violation of the UN Charter; incumbent UN Secretary-General Kofi Annan declared explicitly that he considered the war to be illegal. The invocation of the implied authorization doctrine has arguably had a chilling effect on the taking of action by the Security Council as member states have become wary of resolutions that could be read as impliedly authorizing the use of force.

Short of authorizing the use of force, the Security Council may make other orders to restore or maintain international peace and security. It has levied sanctions and embargoes variously against racist regimes in Southern Rhodesia and South Africa, to attempt (unsuccessfully) to suppress growing civil wars in Yugoslavia and Rwanda, over concerns about the proliferation of nuclear weapons in North Korea and Iran, and to prevent the export of diamonds from Angola and Liberia, amongst many other examples. UN peacekeeping forces, recognizable by their distinctive light blue helmets, are often employed around the world to prevent conflict, assist in the provision of humanitarian aid, and assist the implementation of peace agreements. The mandate extended to such missions varies significantly, from permitting personnel only to carry light arms and use force strictly in self-defense, to broader mandates which euphemistically refer to “peace enforcement.” Famous and infamous peacekeeping missions include those in Rwanda, the former Yugoslavia, the Democratic Republic of the Congo, Somalia, and Sudan.

In 2005, the United Nations General Assembly unanimously adopted World Summit Outcome Document, which endorsed the Responsibility to Protect doctrine as a commitment to protect populations from genocide, war crimes, ethnic cleansing, and crimes against humanity. The RTP (or “R2P”) is built on three pillars, namely the enduring responsibility of each state to protect its population, the commitment of the international community to assist states in meeting those obligations, and the responsibility of states to respond collectively in a timely and decisive manner when a state is manifestly failing to provide such protection. States declared their willingness to take collective action through the Security Council under Chapter VII on a case-by-case basis if the responsibility to protect demanded it. The Security Council first referred to RTP in a 2006 resolution on the protection of civilians in armed conflict, and has since done so in resolutions relating to Côte d’Ivoire, South Sudan, and Yemen.


Consequences Of Breaching The Prohibition

At the conclusion of World War II, the Nuremberg Trials saw convictions of Nazi personnel for crimes against peace—what we today call the crime of aggression, for serious breaches of the prohibition on the use of force. The crime of peace was defined in the Nuremberg Charter (which also criminalized war crimes and crimes against humanity) as “planning, preparation, initiation or waging of a war of aggression, or a war in violation of international treaties, agreements or assurances.” Sixteen defendants were charged at Nuremberg with crimes against peace; 12 were convicted. Japanese leaders were convicted on eight counts of crimes against peace under the near-identical definition in the statute of the International Military Tribunal for the Far East.

Since the codification of the prohibition on the use of force in the UN Charter, there have been very few prosecutions for the crime of aggression. It has proven difficult to reach a consensus in criminalizing it under international law because, well… it’s entirely possible that various influential world leaders over the years have had a personal interest in not doing so.


When the states of the world came together to draft the historic Rome Statute of the International Criminal Court (ICC) in 1998, they agreed that the crime of aggression would be one of the four crimes within the jurisdiction of the Court (along with genocide, war crimes, and crimes against humanity). However, they could not agree on a collectively acceptable definition of the crime, and so delayed activation of jurisdiction over the crime of aggression until consensus could be reached. The 2010 Kampala amendments reached this compromise, defining acts of aggression in terms set out in the 1974 United Nations General Assembly resolution “Definition of Aggressionas:

the planning, preparation, initiation or execution, by a person in a position effectively to exercise control over or to direct the political or military action of a State, of an act of aggression which, by its character, gravity and scale, constitutes a manifest violation of the Charter of the United Nations.

The ability of the ICC to exercise of jurisdiction over the crime of aggression is limited compared to the other ICC crimes in respect of crimes referred to it by state parties and investigations initiated by the Prosecutor of the ICC (though not in respect of cases referred to it by the Security Council), and states can choose to opt out of the Kampala amendments. The ICC can exercise jurisdiction over the crime aggression from July 17, 2018, 20 years to the day after the adoption of the Rome Statute.


While the most effective way to enforce the prohibition on the use of force would clearly be to routinely hold world leaders individually criminally liable for violations, the nations of the world do nevertheless generally observe the prohibition and fight for its preservation. There are already plenty of reasons for states to want to avoid war—they are deadly, destructive, expensive, can be politically unpopular, and they may be lost. Leaders may fear that any erosion of the prohibition could eventually lead to their nation being attacked.

The international legal prohibition on the use of force provides the legal hook on which a lot of additional real-world deterrence hangs. It underpins collective defense treaties which aim to dissuade states from attacking coalitions that would together defeat it. It provides concrete justification for the imposition of sanctions and embargoes against belligerent states. It can justify the suspension of diplomatic relations and exclusion in the international arena—Russia was expelled from the Group of 8 in 2014 explicitly because of its annexation of Crimea (the only significant belligerent annexation of territory in decades), and Turkey’s application to membership of the European Union is being shunned because of its disregard of the rule of law and use of violence within its borders. States that breach the prohibition arguably have their influence and authority in the world diminished as their critics can dismiss their rhetoric with reference to past violations.


For all these reasons, states vigorously argue the legal merits of any uses of force in international arenas like the Security Council, the General Assembly, and the International Court of Justice, condemning or defending actual or perceived breaches or lawful uses of force (though of course they often do so selectively according to their relationship with the nations involved).

Widely-enforced individual criminal liability for the crime of aggression would further bolster the framework of deterrence. It’s not hard to imagine world leaders thinking more carefully about initiating an armed conflict if they risked life in prison by doing so. The prohibition against the use of armed force is an imperative weapon in the fight to keep our world safe. Strengthening the prohibition beginning with the Kampala amendments can only push the world in the right direction toward peace.

Toward A New Exception? 

For many years, various commentators have been arguing in favor of a new exception to the prohibition to permit the use of force on the grounds of humanitarian intervention. A deadlocked Security Council may mean that a government could remain free to wreak unspeakable havoc on its population, safe in the knowledge that no other state’s right to self-defense would be triggered and that the veil of sovereignty would protect it from intervention from the outside world—a scenario that has occurred numerous times since the imposition of the UN Charter.


A norm (or rule) of customary international law is said to have crystallized in the presence of sufficiently widespread and uniform state practice, and the opinio iuris (Latin for “legal opinion”) of states that the practice is motivated by legal duty (rather than by courtesy, convenience, or tradition).

The argument is therefore made that a new norm of customary international law should be recognized which permits states to use force against another for the purposes of humanitarian intervention. UN Secretary-General Kofi Annan said in 1999, “emerging slowly, but I believe surely, is an international norm against the violent repression of minorities that will and must take precedence over concerns of State sovereignty.”

The United Kingdom is the most vocal proponent of the doctrine, and explicitly argues that it considers that a norm of customary international law has crystallized which allows it to use force on an exceptional basis if the following conditions are met:


  1. There is convincing evidence, generally accepted by the international community as a whole, of extreme humanitarian distress on a large scale, requiring immediate and urgent relief.
  2. It is objectively clear that there is no practicable alternative to the use of force if lives are to be saved.
  3. The proposed use of force is necessary and proportionate to the aim of relief of humanitarian suffering and is strictly limited in time and in scope to that aim.


The U.K. states that it has relied on this doctrine in protecting the Kurds in Northern Iraq in 1991, in maintaining no-fly zones in Iraq from 1991, in using force against Yugoslavia in Kosovo in 1999, and in striking Syria in 2018.

Belgium argued a different formulation of the doctrine at the International Court of Justice to justify NATO’s use of force in Kosovo in 1999. It argued that there was an obligation to intervene to prevent a humanitarian catastrophe and that the intervention was intended to save a population in danger, and not an intervention directed at the territorial integrity or political independence of Yugoslavia for the purposes of Article 2(4) of the UN Charter. Belgium invoked precedents of India in using force in Bangladesh in 1971 to assist in securing independence from Pakistan, Tanzania in using force in Uganda in 1979 to overthrow Idi Amin, and Vietnam in using force in Cambodia in 1978 to overthrow Pol Pot. None of the other nine NATO members that were parties to the case argued that humanitarian intervention was a permissible basis on which to use force under international law, and the ICJ ultimately did not rule on the merits of the case.

The fabled third exception has not yet crystallized as a norm of customary international law. The requisite opinio iuris simply isn’t there, whether or not the requisite state practice is. Humanitarian intervention is explicitly opposed as a new exception by influential states such as Russia and China and by coalitions such as the Non-Aligned Movement and the Group of 77. No state has consistently supported the United Kingdom’s stance, and other states that may have benefited from arguing their actions were justified by the doctrine have notably not done so. Customary international law is what states make it, and if the nations of the world want a new exception to the prohibition, they’ll need to argue for it, and a broader consensus is needed before a right of humanitarian intervention becomes a reality.


The other articles in this series deal with international humanitarian law (IHL), the laws of ius in bello—Latin for “rights in war.” These laws are to be contrasted with the international laws governing the use of armed force discussed in this article, the laws of ius ad bellum—the “right to war” (or, as it is often phrased, ius contra bellum, the “right against war”). The two sets of laws are completely separate and independent—the legitimacy or illegitimacy of a use of force cannot absolve a party of its obligations under IHL or deprive any person of the protection of the laws of IHL. The application of the Geneva Conventions is the same regardless of whether a party entered the conflict lawfully or not. (A handy way to remember which is which by noting that the ius ‘in’ bello relates to the laws ‘in’ a conflict.)

It might be the case that a belligerent campaign is unlawful under one paradigm but not the other. The bombing of an enemy nation might constitute a prohibited use of force under the UN Charter in breach of the ius ad bellum, but if the bombing is proportionate and directed only at military objectives, it would not violate laws of the ius in bello. Conversely, a nation may be authorized to use force against another (say, in self-defense) in conformity with the UN Charter, but if a specific attack is directed at the civilian population it would be a war crime under IHL.

As with all of international law, the laws on the use of force apply equally to all states. The prima facie prohibition on the use of force is designed to keep us all safe—I personally prefer that other states are not permitted by law to wantonly attack or invade my country. While you might think your nation should be able to attack another nation just because you don’t like the guy running it, consider what that would mean that other nations could do the same to yours simply if they feel the same way. Imagining your homeland on the receiving end of a use of force is the quickest way to finding wisdom in the prohibition laid down in the UN Charter.


The UN Charter is the most important, but not the only, legal instrument impacting the wider context on the use of force. Many nations’ constitutions address the power to wage war—for example, the U.S. constitution grants Congress the power to declare war, while the constitutions of Japan and Italy reject war as a means for the settlement of international disputes. While domestic laws are hugely important in shaping states’ actions in the international arena, a state cannot invoke its internal law as justification for breaching an international treaty law obligation. Bilateral and multilateral international agreements give further shape to the international collective defense system—for example, the NATO treaty, the now-defunct Warsaw Pact, and the 1961 mutual defense treaty between China and North Korea all provide that their members will act in collective self-defense in aid of any other member which is the subject of attack, in the hopes of deterring any attack on any of them.

A collective security system which restricts the use of force is a system worth protecting and strengthening. For all the United Nations’ faults, the collective security system schematized under its governance has been humankind’s most successful cooperative attempt to limit the scourge of war in history.

The next installment of this series will cover the Principle of Distinction.