The Use of Force in International Law

On the Historical Evolution and Actual Content of the Prohibition

Elaboration, 2019

53 Pages, Grade: 1


Table of contents

1. Introduction. The most important rule in international law

2. Notes on the historical evolution of the prohibition of the use of armed force

3. Nature of the prohibited force. Armed force and economic coercion

4. The exceptions to the prohibition of the use of armed force: the collective security system

5. (continued): The exceptions to the prohibition of the use of armed force: peace-keeping missions and their consensual foundation

6. (continued): The exceptions to the prohibition of the use of armed force: individual and collective self-defence and the resolutions of the Security Council authorizing the use of armed force
A) Collective self-defence
B) Implied powers
C) Modification of the Charter by subsequent practice
D) Use of regional arrangements

7. (continues): The exceptions to the prohibition of the use of armed force: some considerations on humanitarian intervention.


1. Introduction. The most important rule in international law

Both the common sense and many scholars with historical or political background – therefore without expertise in international law – approach international law with the prejudice that war, whose presence is witnessed throughout the history as an element which cannot be eliminated from human affairs, would be a tool which States can still and always legitimately use. War and more generally the possibility of resorting to armed force would represent the counter-proof of the thesis which considers the international society as an example of the state of nature, of the war condition of all against all: the hobbesian condition of homo homini lupusevery man is a wolf for any other man »).

Despite the fact that history records many cases of resort to armed force in international relations – that is, in the community of those entities characterized by the principle of sovereign equality – the consideration of States’ practice in international law does not allow to conclude that in general the use of armed force in international relation is permitted. Quite on the contrary, an adequate analysis of the current international order demonstrates armed force is prohibited as a principle, with the sole exception of self-defence, and that recently such a prohibition has assumed peremptory character. Because of the devastating effects which, at the time of atomic and mass destruction weapons, the use of armed force could produce on the possibility itself of the coexistence of the international subjects, the prohibition of the use of force has become the most important rule in international law and its respect is one of the most important factors which guarantee the coexistence of States and ultimately the very survival of the human race.

2. Notes on the historical evolution of the prohibition of the use of armed force

Even scholars willing to extend the hypotheses of exception to the prohibition of the use of armed force currently in force1 cannot disregard that since its arising from the dissolution of the Respublica of Christian peoples, international law has increasingly provided for limitations of the use of arms2. First of all, the practice of aggression has always been considered illegal, unless it was justified for sanctioning purposes. Since the dawning of international law, a distinction has been established between «just war» (bellum iustum), admitted as a reaction to an illegal act, and «unjust war» (bellum iniustum)3, considered in itself illegal. For Grotius, in substance, «just war» means execution of the law, that is, an instrument for reacting to a wrong suffered, and in general it can be used only when it is strictly necessary4. Considering that the text of Grotius dates back to 1625 and by tracing the official birth of international law back to 1648, it is as if international law since ever provided the prohibition of the use of force at least for aggressive purposes; it is as if the limitation of the use of force was inherent in international law. With the birth of the international society, peaceful coexistence was immediately considered not as a mere «absence of armed conflict» (as a secondary effect of a social contract which has somehow put an end to a more original and authentic conflict dimension), but as a «condition which characterizes the normality of international relations»5, with respect to which, in this initial historical phase, war is both a breach of peace (as «unjust war») and an instrument of peace (as «just war»).

Furthermore, it must be stressed that, since the dawning of international law, war can be unjust not only when it constitutes the implementation of an unjustified aggressive policy towards another international subject, but also when it takes the concrete form of a disproportionate measure. The requirement of proportionality characterises immediately the rules on the possibility of using the armed force: even if the use of force is justified as a sanction, it ceases to be lawful whenever it exceeds the proportion with respect to the offense suffered. The requirement of proportionality is thus harboured at the root of the possibilities of resorting to armed force: the need to fit the sanction to the unlawful act is affirmed at the origin of international law as the raison d'être of the set of legal rules which guarantee the coexistence between international subjects, that is to say those concerning international responsibility – probably the most delicate area of ​​the entire international order. «Proportion», « raison d’être », that is « rational », « ratio » of the norm: it is no coincidence that these features of the most important legal rule of international law are all possible translations of the Greek word lógoj (logos) in which they are gathered together. Reflecting on the historical origin of the limits to the use of armed force, we grasp more deeply the intimate connection between lógoj and law.

From the preceding considerations, the common feeling that in international law, at least as a «just» measure, war is a lawful – or at least a permitted – instrument, could at first sight find some support; but a deeper consideration of the contemporary international rules on the matter allows to conclude that not only the notion of just war is no longer admitted in international law, but also that the possibility to resort to force is in principle banned from the international legal order. If for a relatively long lapse of time (until the middle of the nineteenth century) the notion of just war, in the above-mentioned sense, is not even put in doubt in the relations between States, the improvement of the war instruments (in particular the availability of long-range weapons) gives rise to the need to further limit not only the jus ad bellum (i.e. the legitimate possibility of resorting to armed force, that is to say the just war), but also the jus in bello, that is the concrete possibilities of the right to use weapons (which kind of weapons and against whom) once the conflict has broken out. Starting from the last decades of the nineteenth century, States begin to conclude a series of increasingly numerous agreements aimed, for example, at excluding the use of armed force against the civilian population, the wounded, and prisoners of war: we can think to the Geneva Convention of August 22, 1864 for the Amelioration of the Condition of the Wounded in Armies in the Field, later replaced by the Geneva Convention of July 6, 1906 for the Amelioration of the Condition of the Wounded and Sick in Armies in the Field; the Petersburg Declaration of 1868 Renouncing the Use, in Time of War, of Explosive Projectiles Under 400 Grammes Weight; the II Hague Convention of 29 July 1899 regarding the Laws and Customs of War on Land.

As in the second decade of the twentieth century, the first great conflict had as a consequence that blood was shed all over the world, at its conclusion States realize the opportunity to limit in itself the recourse to war, i.e. also to the «just» war (although, as we will see, without prejudice to the right of legitimate defence, that is the right to resort to armed force in order to repel an armed attack). From this point of view, at the end of the First World War, the Treaty of Versailles of 1919 establishing the League of Nations represents an epochal turning point. It is the first time that States create an international organization focused on the commitment of the Member States to refrain themselves from the use of armed force and to take into account what a common body will establish in this regard. According to Article 10 of the Covenant of the League of Nations,

«[the] The Members of the League undertake to respect and preserve as against external aggression the territorial integrity and existing political independence of all Members of the League. In case of any such aggression or in case of any threat or danger of such aggression the Council shall advise upon the means by which this obligation shall be fulfilled».

However, although the League is highly innovative in its scope, it is marked by a structural weakness constituted by the fact that the right to resort to the just war (i.e. the war for sanctioning purposes) undergoes in practice only a procedural limitation. In this regard, Article. 12 of the Covenant of the League of Nations establishes that

«[the] Members of the League agree that if there should arise between them any dispute likely to lead to a rupture, they will submit the matter either to arbitration or to inquiry by the Council, and they agree in no case to resort to war until three months after the award by the arbitrators or the report by the Council».

The Covenant of the League of Nations, therefore, does not in itself prohibit resort to war as a method of settlement of international disputes, but only sets a time limit of three months in order to induce the State intending to wage war to ponder adequately on its intention. Furthermore, it should be noted that the notion of «war» is an international legal concept which embraces the situation which takes place following a specific declaration, while other, for instance less serious modalities of resorting to armed force not preceded by a war declaration, cannot be interpreted as «war» (for example armed reprisals and some self-defence operations on the borderline between the state of necessity and self-defence), remaining, therefore, as a kind of measure, fully legitimate even under the terms of the Covenant. In short, the Covenant does not limit the use of armed force for sanctioning purposes, but only the war as declared war; furthermore the limit does not encounter war itself, as a measure considered under a substantial point of view, but only places a temporal condition (the lapse of three months from the judicial decision or arbitration authority or the report of the Council of the League of Nations) to make use of it.

A further step forward in the evolution of the matter, is represented by the Pact of Paris of 27 August 1928, also known as the Briand-Kellogg Pact (due to the names of the French and US representatives participating in the negotiation as plenipotentiaries). The Pact of Paris represents a progress with respect to the Covenant of the League of Nations, because it prohibits war tout court as a means of resolving disputes and as an instrument of international policy. In this regard, Article 1 of the Pact of Paris states:

«[t]he High Contracting Parties solemnly declare in the names of their respective peoples that they condemn recourse to war for the solution of international controversies, and renounce it, as an instrument of national policy in their relations with one another».

But even in this case we have a twofold weakness. On the one hand, only the resort to war, and not to every kind of use of armed force, is prohibited (therefore armed reprisals remain lawful, as indeed attested by the practice of those years; as well as all those methods of resorting to armed force not falling within the notion of war); on the other hand, no sanctioning mechanism alternative to the war is established, particularly no restriction in the use of armed force is provided for in the case of violation of the prohibition to refrain from resorting to war. Despite the fact that, within 1939, 63 States had adhered to the Pact of Paris, the convulsive thirties of the twentieth century testify the fragility of this system and quickly ruined towards the second great world conflict. If we did not go beyond the first months of 1945, the harshness of the Second World War and the very serious violations of human rights perpetrated on that occasion would perhaps authorize us to conclude that conflict really represents the deepest essence of human relations without residues and that any attempt to limit this evil, wild, aggressive feature that marks us indelibly is vain.

It is so much more surprising than in the spring of 1945, in San Francisco, just as the Second World War with all its tragedies was coming to an end, something completely opposite to that feature is devised, namely the creation of an international organization which is not only founded on respect for international law in general, but also finally enshrines effectively the prohibition of using armed force. Article 2§4 of the Charter of the United Nations, thus, represents the cornerstone of the evolution of the matter, and at the same time the greatest effort to overcome the previous failures: not the acceptance of our familiarity with war and destruction, but the most serious attempt, if not of their elimination at least of their containment through the clear, very broad statement of the prohibition of the use of force in the following terms:

«[a]ll 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 can be seen from the text of Article 2§4 just mentioned, not only the actual use of force is prohibited, but also its threat; and the concept of prohibited force is intended in a very wide sense, since not only «armed» force is forbidden but «force» tout court.

But before asking whether the term «force» in Article 2§4 embraces other form of measures than the armed ones, we can first remain on the question of armed force, undoubtedly. Through Article 2§4 an attempt was made to formulate the prohibition of use of armed force in the broadest and most complete manner, thus trying to repair the defects of the previous agreements: thanks to Article 2§4 of the Charter any kind of armed measure – other than self-defence, as we will see – adopted unilaterally is now prohibited and prima facie it amounts to an internationally wrongful act. In fact, Article 2§4 is not limited to the possibility to wage war, but the prohibited force is intended in a the widest sense. It means, for instance, that in the framework of the UN Charter, armed reprisals – typical international unilateral sanctions, which could be conceived as a sort of «just war» seen before – are no longer admitted. On the other hand, the banning of the possibility to resort to armed force unilaterally, is balanced by the creation of an alternative mechanism operating in a concerted manner which, thanks to its sophisticated decision-making process, should protect against the abusive use of weapons: namely the collective security system, whose heart is represented by the Security Council. In other words, if until the first half of the twentieth century, States could use armed force for both defence and sanction purposes, the Organization of United Nations operates a sort of a split with respect to the exercise of this faculty: the mechanism of collective security, on the one hand, subtracts from the individual States the faculty of resorting to force (except for self-defence), and, on the other hand, treasuring the experience that it is perhaps not realistic in an anarchist community, like the international society, even to hope to completely annul the faculty of unilateral recourse to the armed force, by establishing the centralization of the management of crises of a certain gravity under the Security Council which, due to its composition and the operation conditioned by the veto power of the five permanent members (United States, Soviet Union, Great Britain, France, China), such a system should ensure special prudence in the decision on the possible use of armed force. The centralization of crises on a body composed of several States should not only remove the abuses that are always present in a unilateral evaluation of the situation, but would correspond in some ways to the desire to make the international society – interpreted by many as «primitive», as it is grounded on self-defence mechanisms – evolve towards that process of centralization which characterizes the evolution of the State, conceived, according to a vision of Hegelian memory, as the most complete institution to which human life caught in its relational aspect could ever hope for.

The system outlined in San Francisco provided that the Member States should make available to the Security Council armies and military means in ways that would be determined in ad hoc agreements (Article 43 of the Charter). Such armed forces should constitute the United Nations army under the command of a Military Staff Committee formed by the Chiefs of Staff of the five permanent members and directed by the same Security Council. The removal from the States of the faculty of unilaterally resorting to armed force was therefore counterbalanced by the provision of this collective security system equipped with its own armed forces. Now, as known, such a military mechanism has never been implemented. Not only the ad hoc agreements which should place military assets at the disposal of the Security Council have never been stipulated, but also for a long time the decision-making mechanism of the vetoes has almost paralyzed the activity of the Security Council itself. Nevertheless, this failure of the collective security system was not followed by the revival, for the individual States, of the full faculty to use the armed force as envisaged by international law in the period prior to the Second World War. Not without surprise – if we consider that the international society does not have authoritative mechanisms –, the States have not claimed, as perhaps one would have expected, the restoration of the power to use armed force, but have even pushed the prohibition referred to in Article 2§4 beyond the contractual limits of the United Nations system and have not hesitated to make it – firstly – a general, customary rule, therefore binding also for the non-member States, and – then, quickly, within a few decades – even a peremptory rule.

The prohibition to resort to force has therefore consolidated as the most important norm of current international law and the only exception allowed is self-defence. Of course, with this assumption, we do not want to disregard the number of occasions in which, over more than half a century of the life of the United Nations, the prohibition has been violated. As we will see later, the issue of the limits to the use of armed force has received new attention when, with the end of the Cold War, the Security Council began to function – often in a manner that does not comply with the Charter; and especially after the attack on the Twin Towers in 2001, part of the doctrine sought to enlarge the scope of the possible exceptions to the prohibition of the use of armed force in order to have more effective means in the fight against international terrorism.

However, these attempts to enlarge the scope of the only exception to the prohibition of the use of armed force, in other words the attempts to widen the limits of admissibility of self-defence, have proved to be not only unconvincing, for they have been criticized by the States, but also, from a logical point of view, they do nothing but confirm the full validity and cogency of the prohibition in question. As noted by the International Court of Justice in the case Military and paramilitary activities in and against Nicaragua (§186), precisely with regard to the rules prohibiting the use of force, «[i]t is not to be expected that in the practice of States the application of the rules in question should have been perfect, in the sense that States should have refrained, with complete consistency, from the use of force or from intervention in each other's interna1 affairs. The Court does not consider that, for a rule to be established as customary, the corresponding practice must be in absolutely rigorous conformity with the rule. In order to deduce the existence of customary rules, the Court deems it sufficient that the conduct of States should, in general, be consistent with such rules, and that instances of State conduct inconsistent with a given rule should generally have been treated as breaches of that rule, not as indications of the recognition of a new rule. If a State acts in a way prima facie incompatible with a recognized rule, but defends its conduct by appealing to exceptions or justifications contained within the rule itself, then whether or not the State's conduct is in fact justifiable on that basis, the significance of that attitude is to confirm rather than to weaken the rule».

As can be seen from this clear reasoning, the presence of elements of the practice contrary to the prohibition of the use of armed force are not configured in such a way as to seriously cast doubt on the fact that this rule is still fully in force in international law and indeed constitutes the norm most important among those that compose the same jus cogens.

To confirm what we have seen so far, we can think only of the very recent affair involving Russia and Ukraine6. On the one hand, some Western powers (the United States and the European Union) considered the behaviour of Russia to amount to an act of aggression, but with regard to this they expressed the firm intention of resorting only to economic measures (to confirm of the fact that armed measures are banned by international law). For its part, probably with the purpose to minimize its responsibility in the affair, Russia has affirmed (in a manner not far from being a pretext) that the Russian troops penetrated into Ukraine would have acted on voluntary basis and not as organs of the Russian State, thus declining the own responsibility for the act of aggression and at the same time confirming the full validity of the prohibition of the use of armed force in international law.

3. Nature of the prohibited force. Armed force and economic coercion

Since the Security Council began to «function», from 1990 onward, the debate on the use of force has interpreted the term «force» essentially as meaning «armed force». Nevertheless it must be recalled that, some years before the fall of the Berlin wall, some scholars, considering the serious economic crisis that occurred during the seventies, had opened the question whether the forbidden force should be limited only to the use of arms, or extended to coercive measures of different nature, for example political or economic force, at least when it took the form of measures of considerable gravity7. This broadening of the notion of forbidden force had found the support by the developing countries and indeed, given the growing interdependence between States, as a consequence, it allows to consider as in substance analogous to an act of armed aggression the situation suffered by a State which, depending economically in everything and for everything from another, is forced by the latter through economic measures to behaviours which it would never adopt in a different situation. An economic strangulation could essentially materialize a violation of the sovereignty of the State or of its political independence. This orientation, moreover, lost importance when, with the end of the Cold War, the practice of the Security Council brought to the fore the issue of armed force and the limits of its use. Nevertheless, in the contemporary society tending towards total globalization from an economic point of view, it is perhaps not unlikely to expect within a short time a resumption of this attempt to extend the notion of forbidden force even to economic measures of a significant gravity, in order to protect more effectively the sovereignty and independence of States. More recently, in the context of the United Nations, the question has arisen whether the economic measures decided by the Security Council must not infringe a minimum level for the survival of the civilian population and whether they may result in a violation of the economic position of States to which the measures adopted are not direct. Someone8 argues that, following the entry into force of the Charter in the UN, such questions would have given rise to the consolidation of a customary norm limiting the resort to economic measures. Actually, it seems to me that, in the terms in which the question seems to have been posed so far, it is an application of the general limits of the «principles of humanity» which, already before the entry into force of the Charter, were envisaged for the adoption of reprisals. Although measures of the UN Security Council are not properly «sanctions», this limitation of the principles of humanity seems to have been grafted onto international law since long time, since the first attempts to regulate the jus in bello. Another issue is whether, in general, the use of prohibited force should be understood in an broad sense, also including economic coercion to the extent that it can be considered as equivalent to armed force, which should lead to consider as illegal even retortions (that is unfriendly retaliatory acts, not amounting to a violation of international law) of a considerable gravity (for example, the refusal to enter into a commercial treaty – what a State would always be free to do – with a State whose economy totally depends on the exchanges which could be carried out thanks to the treaty to stipulate).

4. The exceptions to the prohibition of the use of armed force: the collective security system

Once it is clarified that both the threat and the use of armed force are prohibited in international law, it is necessary to dwell on the exceptions admitted to this prohibition. In this regard, a first distinction must be drawn according to whether the permitted armed force materializes in a measure adopted by a State unilaterally, or it is a measure adopted by the Security Council in the exercise of its responsibility for maintaining and restoring international peace and security. Given that the system envisaged by the Charter is based on this sort of counterbalance, that is to say, the attempt to centralize the use of force through the security system is sourced from the subtraction from the States of the general faculty of threatening or using armed force. collective, I will first outline this system briefly.

The maintenance of international peace and security is the primary responsibility of the Security Council (Article 24 of the Charter). Although many provisions concerning the Council are spread in various parts of the United Nations Charter (on the other hand, the Council is the most important body of the UN, firstly because its decisions are mandatory for Member States), it can be affirmed that the duties of the Council with regard to the maintenance of international peace and security are essentially gathered in two fundamental Chapters of the Charter, Chapter VI and Chapter VII.

Chapter VI (including Articles 33 to 38) is dedicated to the peaceful settlement of disputes. Examining the provisions of this Chapter, it is clear that in matters of dispute settlement the Security Council can only address recommendations to States, that is non-binding resolutions. The prerequisite for the adoption of such recommendations is that there is a controversy or a situation of a certain gravity: in order for the Council to be able to take action based on Chapter VI the dispute or the situation must be such that its continuation is «likely to endanger the maintenance of international peace and security» (Article 33 of the Charter). In such a case the Council can play a mediating role, that is, it can invite the States parties of the dispute to adopt the so-called peaceful means for dispute settlement; or, pursuant to art. 37§2, it can indicate «terms of settlement», that is it can suggest the parties a solution on the merits. So briefly: on its own initiative, the Council can consider a dispute between States: 1) only if the dispute presents certain traits of gravity; 2) and adopting only recommendations (regarding the procedure or the merits), but never imposing a binding decision on the States involved. Only if the parties expressly request it, the Council may make recommendations regarding a dispute regardless of its seriousness (Article 38 of the Charter), but even in this case it cannot adopt mandatory resolutions (i.e. decisions).

Therefore, as regards the solution of international disputes or situations of a certain gravity as indicated in Article 33 mentioned above, the Council can only adopt recommendations and not decisions, i.e. it cannot impose a solution on the merits according to which the dispute should be composed, nor indicate in a compulsory manner for the parties which means of settlement they shall resort to. At the most, the Council can remind the parties that legal disputes must be referred to the International Court of Justice, or it can confirm their obligation of peaceful settlement in force under Article 2§3 of the Charter by inviting the parties to adopt such means. The clarity of the Charter on the nature of the Council’s competences in the field of settlement of disputes and/or situations is not surprising: the Council is a political body and has neither the time of reflection, nor the skills, nor the specialist knowledge, not even the experience and the impartiality of a judge. Its function is that of a policeman who tries to avoid the aggravation of a certain dispute or situation which could lead to an escalation dangerous for the entire planet. I insist on this point because, instead, during the nineties we had an overlapping of functions of the Council (between Chapter VI and Chapter VII) in this matter.

But we shall proceed step by step.

If under Chapter VI the Council can only adopt recommendations, its powers are much more significant pursuant to Chapter VII, dedicated to action to protect (= to maintain and/or restore) peace and in which the Council can address States compulsory decisions.

Chapter VII opens with Article 39, according to which: «[t]he 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». So, firstly, Article 39 of the Charter indicates the conditions legitimating the action of the Council: but threat to peace, breach of the peace or act of aggression are three hypotheses which the Charter does not define further, but which are intended as more serious in comparison with those envisaged in Chapter VI. Regardless – for the moment – of how these three conditions shall be interpreted, the occurrence of one of them, pursuant to the same Article 39, allows the Council to recommend and/or decide peaceful measures (exemplified in Article 41) and/or measures involving the use of armed force (exemplified in Article 42).


1 For instance W. Michael Reisman, «Present Problems of the Use of Force in International Law, B. Sub–group on Humanitarian Intervention», report presented to the Institut de Droit International, Santiago Session 2007, Pedone, Paris 2007, pp. 171–201, at 200 f.

2 See Benedetto Conforti, Comment on Reisman, Institut de Droit International, Santiago Session 2007, Pedone, Paris 2007, pp. 201-207.

3 Hugo Grotius, De jure belli ac pacis. Libri tres (1625), Officina Wetsteniana, Amsterdam reprint 1712, Liber I, Caput I, § III, p. 3.

4 Hugo Grotius, De jure belli ac pacis, Liber II, Caput XXIV, § IX, p. 615

5 Giorgio Badiali Il diritto di pace di Alberico Gentili, Il Sirente, Fagnano Alto 2010, p. 6.

6 Especially in the developments up to the end of August 2014, on which see Maurizio Arcari «La crisi in Crimea», in Diritti umani e Diritto internazionale, vol. 8, n. 2, 2014, pp. 473-479.

7 See for instance Richard B. Lillich, «Economic Coercion and the International Legal Order», in International Affairs, vol. 51, No. 3, 1975, pp. 358–371.

8 Benedetto Conforti, Carlo Focarelli, Le Nazioni Unite, IX ed., Cedam, Padova 2012, p. 267.

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The Use of Force in International Law
On the Historical Evolution and Actual Content of the Prohibition
University of Rome "La Sapienza"
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Dr. Giuliana Scotto (Author), 2019, The Use of Force in International Law, Munich, GRIN Verlag,


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