The use of force by one country against another is one of the most ancient forms of international action. Before states, even before geographic countries, tribal gatherings of humans would seek to assert their will over other peoples through the use of violence. As time went by, diplomacy developed, ambassadors were sent out, messengers carried treaties of peace, agreements were signed, borders were defined, but still, when all was said and done, every country still felt that it had an inherent right to use force against its neighbours if it so wished. Indeed, the history of diplomacy is, albeit simplistically, the history of countries' attempts not to be attacked by others, in many cases so that they might successfully launch attacks of their own.


Treaties are the oldest means of making conflict illegal. Binding upon all parties, and backed up by marriages, gifts, tributes and ultimately force of arms, their object was to avoid conflict by simply agreeing not to fight; usually in return for some benefits. Of course, promises are made to be broken and treaties were not very successful: the peaces they brokered were largely short term, even those that were in name "universal and perpetual." The true use to which treaties were put was strategic. They were a means of demonstrating friendship and brotherhood, and although a nation that broke its word would not be so greatly trusted in the future, all states did so so often that it hardly mattered. Machiavellian rulers would delight in making peace only to lower the enemy's guard, and those who placed their faith in the honesty of their fellow men soon cursed their trusting nature.

The result of the diplomatic backstabbing was that war continued pretty much unabated, people died for king and country, killed for scraps of land and cloth, celebrated great victories and suffered terrible defeats. This was all well and good when warfare was looked at abstractly or romantically; when poets and story tellers spoke of the glories of battle, the intellectual rigours of strategy, of noble kings leading grand armies against nefarious and cowering enemies, it is understandable that the leaders of the day became fired up with blood-lust. But entertaining though war was for the nobility, it was becoming less so for the ordinary people. As centuries wore on, the wars became more terrible, the battles less noble and the fighting more bloody. By the turn of the twentieth century, the pacifist movement was taking hold of a considerable minority and the atrocities of the First World War brought about the first serious international attempt to end conflict between nations.

The League of Nations

In 1920 the Paris Peace Conference concluded with the founding of the League of Nations. It was to function on the basis of collective security – there were to be no more power blocs such as those that had led so disastrously to war in 1914, but instead all nations were to be for each other, and should one step out of line, all would act. Under the Covenant of the League of Nations – the originating treaty setting up the organisation – the member states agreed to respect each other's territorial sovereignty and submit any disputes that might lead to war to the Permanent Court of International Justice for arbitration. They further agreed that if any state should go to war, contravening the Covenant, then the members would break off all commercial intercourse with that state and submit the matter to to the League to consider what military action need be taken.

Ultimately though, the League was a weak organisation. The United States of America refused to join it and Japan, the USSR, Italy and Germany all eventually resigned from it. The pacifist attitudes that prevailed throughout the interwar period meant that the member states were reluctant to consent to military action of any kind, and the League's dwindling membership severely lessened the blow of any economic sanctions ordered. When Hitler invaded Czechoslovakia in 1938, Britain and France decided to appease him, thus putting the last nail in the coffin of the League of Nations and cementing the path to the Second World War.

The devastation caused by the Second World War was on a scale never seen before. Whole cities were ruined, millions upon millions were dead, and once again it was the collective opinion of the world that this must never happen again. The League had failed, and its failures would be learned from. In 1945 the United Nations was formed and in 1946, the League was disbanded and its functions turned over to the UN. High amongst these was the duty of preventing the use of force by one state against another, unless it was absolutely necessary.

The United Nations

From the start, the United Nations had a significant advantage over its predecessor: the major powers of China, the USSR, and the USA had all become members. This did not mean, however, that everything went smoothly. Whilst the United Nations sought to prevent aggressive war, article 2(4) of its charter explicitly stating that all member states must refrain from the threat or use of force against the territorial integrity or political independence of any state, it also recognised that there were occasions when violence would erupt and attempted to make provisions for them. Article 51 specifically allowed states to use force in self defence and the Security Council was empowered under Chapter VII to sanction military action in certain circumstances in order to preserve international peace and security.

Problems emerged under both these exceptions. Self defence was subject to legal battles – was it legal to act in pre-emptive self defence? What exactly did self defence entitle a state to do? What if both sides of a conflict claimed they were defending themselves against one another's aggression? The Security Council found itself paralysed as East and West vetoed each other's proposals, not due to the pacifism that rendered the League of Nations powerless, but due to the ideological antagonism of the Cold War. However, over the sixty years that the law has been in force cases before the International Court of Justice have refined the definitions, and the decades since the collapse of communism in Europe have in some cases allowed the Security Council to unify, as it did with Iraq in 1990 and the Kosovo crisis in 1999.

The upshot of these developments is not that war has been made a thing of the past; far from it. What has happened is that conflicts between states now take place within a recognisable legal framework - it is possible to say whether an individual conflict is permitted under international law. This has a slight deterrent effect as states now feel that it is necessary to justify their actions within the law or risk losing their international standing. More importantly however, it allows those who feel that they have been wronged by an international conflict to seek compensation. Furthermore, the difficulty of establishing a legal case for conflict encourages states to try diplomatic methods of reconciliation first, before resorting to force of arms.

The Law Today

As should be pretty clear by now, legally, it is not easy to use force against another state without provocation. The law has evolved from allowing conflict except where specifically forbidden by mutual agreement, to the current state of preventing the use of force in all but a select few permitted circumstances. Currently, there are two such circumstances: self defence, and collective action under Chapter VII of the Charter of the United Nations in order to preserve international peace and security. Peace treaties and cease fire agreements are still very important, but are usually treated with no more respect than their historical counterparts.


There are currently two separate prohibitions of the use of force in international law; the Kellogg-Briand Pact of 1928 and Article 2(4) of the United Nations Charter. These are both legally valid prohibitions, but only the latter is binding on all states.

Kellogg-Briand Pact

Signed by thirty two countries, including all the major European powers, and the major powers of the American continent, this treaty agreed under Article I to renounce war as a solution to international controversies that might arise between them and under Article II mandated that pacific solutions must be found to all disputes. Of course, given the events that took place a decade or so later, it was obviously not a very successful treaty. Nevertheless it has had a rather profound effect on international law. Its prohibition on the use of force was implicitly (and specifically in the case of the United States) taken as not including the use of force in self defence, thus allowing the armed response of the Allies to Hitler's invasion of Poland. Perhaps even more importantly, especially when it came to enforcing the law, the treaty was taken by the Nuremberg tribunals as creating the war crime of Aggression – ending once and for all the right of conquest.

One major criticism of the Pact is that whilst it prohibits the formation of a legal relationship of war between two states, it does not define what war actually means. International law has long recognised the existence of violent actions that do not amount to war (reprisals for example), and it would appear that under the Pact these would still be considered to be legal. This problem has been dealt with however by the Charter of the United Nations (1945) which is now considered to be the foremost prohibition on the use of force in international law.

Article 2(4) Charter of the United Nations

The Charter is considered to be a part of Customary International Law, which, for reasons I will go into in another writeup, is considered to be binding on all states. This effectively means that even if they disagree with it, all states are bound by what it contains. Crucially, this includes article 2(4) which states that:

"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."

The effect of this article is very simple. It makes it illegal under international law to use force against another state. Once again however, there is a problem of definition; what exactly counts as force? Violent invasion of territory certainly does, but what about an economic sanction such as an embargo? The 1970 Declaration on Principles of International Law described the duty as one to avoid "military, political, economic or any other form of coercion aimed against the political independence or territorial integrity of any state." This appears to make it clear that any form of punitive action is illegal, but largely due to the unwillingness of states to give up all their influence over each other, the point remains disputed. It is also important to note that it is not just the use of force that is prohibited, but the threat of force. This raises interesting questions as to the legality of nuclear deterrents, though that is a complicated topic best dealt with in its own writeup.

The above two measures are between them very restrictive. Were they absolutes, it is likely that they could never be effectively enforced. All but the most ardent pacifists argue that in certain limited circumstances the use of force by one or a collection of states against another is legitimate. It is important to remember that the wish to avoid another world war was foremost in the minds of those who drafted these provisions, and yet it is likely none of them would have argued that Hitler's advance across Europe should have been countered by peaceful means alone. Equally, it is difficult to imagine how an aggressive state could be brought back into line without sanctioning some collective military action. For these reasons, International Law acknowledges some exceptions to the general prohibition.


The use of force by one state against another is in fact quite rarely sanctioned. This does not necessarily mean that it is always legal, it merely means that due to the international political scene at the time, the international community was unable to successfully complete the procedures necessary to condemn the violent action. Nevertheless, most states want to be seen as acting within the law and will almost always attempt to justify their actions by reference to the available framework. In particular, with regard to use of force, states will try and bring their actions at least rhetorically within the existing exceptions to the prohibition.

Self Defence

Although the object of the Charter is to work towards a world in which self defence will be unnecessary, it is universally accepted that despite the efforts of the international community, wars will occur and state will find their territories under threat from invasion. It is held by almost everyone that a state under attack has an inherent right to self defence, and even if they did not, it is extremely unlikely that they would lie down and let the invaders in simply because to fight them was against the law. Therefore, the Charter contains Article 51 which states:

"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 "

Unfortunately, the law is not that simple. Although it is a point of common sense that if a state is actually attacked, it should be allowed to defend itself, there remains the question of what it may do in its defence. Is it legitimate, for example, for an invaded country to launch air strikes against major enemy cities in response to a small skirmish on its borders? The answer, according to the law, is no. It is required that all action be no more than necessary and proportionate in the face of the invasion. The seminal case for this principle is The Caroline Case of 1837, the facts of which are as follows:

Canadian rebels on Navy Island declared themselves the Republic of Canada and sought to wage a war of independence against the United Kingdom and Upper Canada. Sympathetic Americans, hoping for a British defeat, filled a boat, the SS Caroline, with armaments and sent it towards the rebellious island. However, Canadian loyalists and the British Navy got wind of the enterprise and sent a militia across the border to capture the Caroline. They were successful and, having evacuated the crew, set the boat on fire and cast it adrift over the Niagara falls.

Naturally this led to a decidedly tricky diplomatic situation. Britain and the United States were still not on excellent terms, having fought a war only twenty five years earlier, and now Britain had sent armed men into American territory, seized an American vessel and spectacularly destroyed it. Ultimately however, the difficulties were resolved by the Webster-Ashburton Treaty 1842. It was decided that the British were acting in self defence. This required the "government to show a necessity of self-defense, instant, overwhelming, leaving no choice of means, no moment for deliberation." Applied to The Caroline Case it is apparent that had the British not acted as they did the Caroline would have made it to the rebels, greatly increasing their ability to threaten British interests, therefore, the British had to act immediately. Notably, despite contemporary American propaganda reports of massive casualties, the boat was in fact empty when it was sent over the falls. Had the British killed everyone aboard without offering them the chance to leave, that would have been disproportionate and therefore an aggressive, rather than defensive, action.

The reader may have noticed however that The Caroline Case significantly pre-dates the 1945 charter and as such it might be argued that the rather stricter wording requires that an armed attack to actually be occurring before self defence becomes a legal use of force. Although this reasoning would be sound in a national common-law system, at an international level treaties do not necessarily overrule previous "customary" law. The Webster-Ashburton Treaty merely codified these already existing customs.

The Nicaragua Case elaborated further on the law of Self Defence, specifically by considering exactly what constituted an "armed attack" for the purposes of the Charter. This was a complicated case involving a lot of complicated and technical areas of international law, but with regard to the Use of Force the relevant facts were as follows:

For some time the United States had been supporting Contra, a right-wing counter-revolutionary movement within Nicaragua by supplying them with arms and training. The case centred on whether or not the US had acted illegally and in particular whether or not their actions constituted an illegal use of force. After lengthy discussion of technical issues of customary and treaty law, the court gave its decision: by assisting Contra, the US was in breach of its legal international obligation of non intervention and its actions were considered to be an illegal use of force against another state.

This case, in conjunction with The Caroline Case raises further issues. In particular, it has been argued, primarily by the United States that they give rise to a right of pre-emptive self defence, particularly in today's post-9/11 climate. The argument is that if a state can be shown to be sponsoring terrorists who have the objective of breaching the territorial integrity of, for example, the US, the US may take aggressive action against that state as a matter of self defence. This line of reasoning was used in part to justify the invasion of Afghanistan in 2001, and has been used unofficially by some commentators to justify the invasion of Iraq, though the questions arising out of that particular conflict are, once again, for another writeup.

Just as it is impractical to suggest that the use of force in self defence should be outlawed, it would be naïve to suggest that without the threat of force, all states would become pacifist overnight. Furthermore, the UN was at least partially founded on the grounds that the horrors that were perpetrated in Nazi Germany should never be allowed to happen again. This meant that the UN was tasked with preventing humanitarian disasters from occurring and putting a stop to those that do. Although it is arguable that as a body, the international community acts in self defence when it takes action against such atrocities, the practice is very different. What should be done about a situation is debated by the Security Council and it is they that decide whether action should be taken and if so, what form it should take. It is notable, however, that should the Council wish, it may authorise the use of force against any country it chooses, in order to maintain or restore international peace and security. The UN does not have an army of its own, and so any fighting is conducted by the states themselves, in a process known as Collective Action.

Collective Action

The United Nations may take collective action under what are known as the Chapter VII provisions. Article 39 briefly sets out how they operate:

"The Security Council shall determine the existence of any threat to the peace, breach of the peace, or act of aggression and shall make recommendations, or decide what measures shall be taken in accordance with Articles 41 and 42, to maintain or restore international peace and security.

The Security Council is therefore only empowered to authorise the use of collective action in circumstances where there is a "threat to the peace" a "breach of the peace" or an "act of aggression." It may determine when such circumstances have arisen on its own authority. It may then determine what to do to rectify the situation. Article 41 refers to non violent coercive measures, such as embargoes, withdrawal of diplomatic relations, cutting off of communications and the like. Article 42 refers to the measures that may be taken should those under article 41 prove inadequate. They are:

"such action by air, sea, or land forces as may be necessary to maintain or restore international peace and security. "

Therefore, provided the non-violent measures under Article 41 have failed, if there is still an ongoing situation under Article 39, the Security Council is empowered to authorise the use of force. It is notable however, that once again the requirement of necessity and proportionality is present; the Council may only order such force as is required to "maintain or restore international peace and security." This means that provided international peace and security have been restored, further action would be illegal – even if there are other non-legal, but moral reasons for continuing with the military action, such as an unpleasant regime.

The power to authorise the use of force against a sovereign state carries with it immense responsibility. Although they must justify their decision within the above articles, the Security Council is relatively free to authorise the use of force when it wants to, and it is by no means certain that it will always do so fairly; partisan considerations may well play a part in deciding whether or not to allow a war. Therefore, in order to ensure that the Council only ever sanctions extreme actions in good faith, safeguards were put in place.

Passing a Resolution

Legally, passing a resolution authorising the use of force is not a complicated process. Ideally it should be drafted in clear, unambiguous language and be agreed on by the entire security council. Unfortunately, in both these respects, political problems have a tenancy to arise.

One of the controversies over the invasion of Iraq in 2003 was whether or not it was duly authorised by Security Council Resolution 1441. France and Russia both claimed that it was not, whereas some legal commentators in the United States and the United Kingdom claimed that it was. The reason for this disagreement was down to the language used. In international diplomatic speak, the phrase "all necessary measures" and very similar constructions are always taken to mean an authorisation of military action. However, 1441 did not include that particular phrase; instead, it referred to "serious consequences." It is accepted in academic circles that this wording was deliberately ambiguous in order to make sure that both sides of the debate would vote in favour of it – each believing their own interpretation was correct.

The reason it is important to ensure that all sides are happy with the wording of a potential resolution is because of the system under which the Security Council operates. In order for a resolution to be enacted it must be accepted not only by a majority of the countries on the council but by all of the permanent members; that is the USA, Russia, France, the UK, and the People's Republic of China. Due to the partisan and suspicious nature of international politics and the tendency for opposing power-blocs to emerge, it can be extremely difficult for states proposing resolutions to secure the agreement of the other members. This is dramatically illustrated by the fact that during forty years of the cold war, only eight hundred or so resolutions were passed, whereas since the fall of the Iron Curtain less than twenty years ago, the number is closer to one thousand!

Once a resolution is passed, it must be enforced by the members of the United Nations. Under Article 43, the Security Council may call upon any member state to contribute armed forces to the action taking place. This is subject to a certain degree of negotiation with the Council and any country called upon to contribute may under article 44 participate in any decisions the Council makes regarding how their forces are to be used. Under Article 49, states can be asked to make mutual contributions to the enforcement effort. Normally, this involves more passive actions such as allowing aircraft to pass through their airspace or ships to resupply at their docks, but in particular this article ensures that states do not actively send troops against those acting on behalf of the United Nations.


There are other technical elements to the law of the Use of Force today. Complex questions over humanitarian intervention abound. The legal nature of peace treaties and cease fires has not been touched on. The use of nuclear weapons has not even been considered, nor have the numerous war crimes that arguably come under this heading. Nevertheless it should be clear that international law as it stands today has placed fetters on the prosecution of war, restricting it only to those circumstances where it is necessary as a response to a threat to the peace. This is, it cannot be denied, a significant step forward. There is still, however, a long way to go. The United Nations was created to preserve the peace, and there is very apparent hypocrisy in its seeking to enforce its will through violence.

It has been suggested that the progression of the international community has been similar to that of nation states – moving from a free-for-all anarchic existence to a code of custom for blood feuds and violent duel, which ultimately gave way to a system of courts with legal remedies and sanctions. It may be hoped that in the future states' respect for international law may grow and the problems that even today are solved through armies, navies and air forces will instead be brought to the courtroom and dealt with not according to military might, but according to the principles of justice and fairness.


Shaw, M N International Law (5th Edition, Cambridge University Press, 2005)
Freeman, M D A Lloyd's Introduction to Jurisprudence
The Caroline Case 1837, 29 Brit & For St Papers 1137
Nicaragua v. United States of America ICJ Reports, 1986 p.14 (noded here
The lectures of Professor R. Piotrowicz, Professor C Harding, and M. Odello and the Seminars of N. Szablewska

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