Doctors without borders | The Practical Guide to Humanitarian Law (2024)

1. United Nations

Article 2(4) of the UN Charter, signed in 1945 in San Francisco, spells out 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,” in accordance with the principle of peaceful settlement of disputes.

Nonetheless, resort to armed force is authorized under two circ*mstances:

1. In the case of self-defence, individual or collective, authorized by article 51 of the UN Charter; and
2. In the context of collective security measures adopted by the UNSC (Art. 42 of the UN Charter).

➔ Peacekeeping ▸ Self-defense

Article 39 of the UN Charter provides that the UNSC is the sole body competent to determine the existence of any threat to the peace, breach of the peace, or act of aggression. However, because no consensus was found regarding the definition of aggression during the first working sessions of the UN General Assembly (UNGA), the issue of the definition was postponed and transmitted to the UN International Law Commission (ILC). The ILC did not reach an agreement on a definition either and concluded in its report to the UNGA in 1951 that aggression, “by its very essence, is not susceptible of definition” (A/CN.4/44, p. 68). Several other special committees were entrusted by the UNGA to propose a definition of aggression, but all failed to agree upon a definition.

It was not until 1974 that the UN unanimously adopted a definition of aggression. Resolution 3314 (XXIX) of the UNGA, which is based on the definition of aggression proposed in 1935 during the Conference for the Reduction and Limitation of Armaments, defines aggression as “the use of armed force by a State against the sovereignty, territorial integrity or political independence of another State, or any other manner inconsistent with the Charter of the United Nations.” The UNGA specified that, in this definition, the term “State” is used without prejudice to questions of recognition or to whether a State is a member of the UN and includes the concept of a “group of States” where appropriate. According to the UN, the “first use of armed force by a State in contravention of the Charter shall constitute prima facie evidence of an act of aggression,” but this act must be of a “sufficient gravity” to be determined as such by the UNSC.

For an act to be considered “aggression,” three criteria must be met. The act must: (1) be perpetrated by a State; (2) involve the use of armed force; and (3) reach a level of sufficient gravity, as defined by the UNSC and must give rise to reactions of self-defence or sanctions imposed by the international community. The definition excludes ideological and economical aggressions and does not provide for the possibility that these acts might be perpetrated by non-State actors (armed groups or other entities).

According to the UNGA Resolution (A/RES/3314 (XXIX)), the following acts constitute acts of aggression, regardless of whether a declaration of war has been made. This list is not exhaustive:

1. The invasion or attack by the armed forces of a State of the territory of another State, or any military occupation, however temporary, resulting from such invasion or attack, or any annexation by the use of force of the territory of another State or part thereof;
2. Bombardment by the armed forces of a State against the territory of another State or the use of any weapons by a State against the territory of another State;
3. The blockade of the ports or coasts of a State by the armed forces of another State;
4. An attack by the armed forces of a State on the land, sea, or air forces, or marine and air fleets, of another State;
5. The use of armed forces of one State that are within the territory of another State with the agreement of the receiving State, in contravention of the conditions provided for in the agreement or any extension of their presence in such territory beyond termination of this agreement;
6. The action of a State in allowing its territory, which is placed at the disposal of another State, to be used by that other State for perpetrating an act of aggression against a third State;
7. The sending by or on behalf of a State of armed bands, groups, irregulars, or mercenaries to carry out acts of armed force against another State of such gravity as to amount to the list of acts listed above, or its substantial involvement therein.

Despite the adoption of this definition of aggression in 1974 by the UNGA, the UNSC continued to use the more neutral terminology of a threat “to peace and international security” or “breach of international peace and security” in its later management of international crises, such as Israel’s successive invasions of Lebanon in March 1978 (S/RES/425) and June 1982 (S/RES/509), or Iraq’s invasion of Kuwait in August 1990 (S/RES/660), even though invasion constitutes an act of aggression according to the definition of the UNGA Resolution 3314.

The terms employed by the UNGA to describe the use of force by the Russian Federation against the territory of Ukraine has changed over the years. Indeed, in 2014, the UNGA did not refer to the term “aggression” in its resolution (A/RES/68/262) regarding the Russian Federation invasion and subsequent “annexation” of the region of Crimea but rather spoke of the disruption of the territorial integrity of Ukraine and requested that all States desist and refrain from actions aimed towards this disruption and to pursue immediately the peaceful resolution of the situation with respect to Ukraine. In contrast, in its 2022 resolution (A/RES/ES-11/1), the UNGA used much stronger language and did not hesitate to use the word aggression and to single out the Russian Federation in its call to immediately cease its use of force against Ukraine and to refrain from any further unlawful threat or use of force against any Member State. It is interesting to note that the UNGA intervened in 2022 following a call for an emergency special session (S/RES/2623(2022)) under a “Uniting for Peace” resolution of the UNSC because of the blockages of the Russian Federation at the UNSC level (using its veto power) from adopting a resolution on the Ukraine situation. This represented the first time in four decades that the UNSC has adopted such a “Uniting for Peace” resolution.

The 2022 UNGA resolution (A/RES/ES-11/1) was adopted by more than the two-third required majority needed for such resolution that relates to important issues such as maintenance of international peace and security (Art. 18(2) of the UN Charter). With this resolution, the UNGA acted not only to assert a moral position that the invasion is wrong, but also to trigger potential political and legal consequences for Russia and its leaders in the framework of State international responsibility as well as individual criminal responsibility.

The concept of a “Uniting for Peace” resolution was first introduced on 3 November 1950 via the UNGA resolution A/RES/377 (V) and is based on article 12 of the UN Charter stipulating that the UNGA can only act on an issue related to maintenance of international peace and security when the UNSC is not playing its role in that matter. The legal responsibility triggered by the Russian Federation aggression against Ukraine have been further elaborated by later UNGA resolutions (A/ES-11/L.6 and A/RES/ES-11/5) recognizing that Russia must be held accountable for its aggression against Ukraine, and which recommended the creation of a registry of damages. It also recognized the need for Russia to afford reparation for injury caused by its violations of international law.

2. International Court of Justice

A number of rulings of the International Court of Justice have also provided definitions of the term “aggression.” In the case concerning military and paramilitary activities in and against Nicaragua ( Nicaragua v. United States of America , June 27, 1986) the International Court of Justice (ICJ) held that “while the concept of an armed attack includes the dispatch by one State of armed bands into the territory of another State, the supply of arms and other support to such bands cannot be equated with armed attack” (para. 247). The Court found that financial support, training, supply of weapons, intelligence, and logistic support constitute a clear breach of the principle of the non-use of force and the principle of non-intervention in the internal affairs of a State, that is, “a form of conduct which is certainly wrongful, but is of lesser gravity than an armed attack” (para. 247). It is to be noted that in the French version of this judgment, the phrase “armed attack” is translated as “ agression armée ”.

Moreover, the Court noted that indirect aggression within the meaning of article 3(g) of Resolution 3314 of the General Assembly shall, in order to be qualified as such, consist in “the sending by or on behalf of a State of armed bands, groups, irregulars or mercenaries, which carry out acts of armed force against another State of such gravity as to amount to (inter alia) an actual armed attack conducted by regular forces, or its substantial involvement therein” (para. 195). The Court considers that this description reflects customary law; “in customary law, the prohibition of armed attacks may apply to the sending by a State of armed bands to the territory of another State, if such an operation, because of its scale and effects, would have been classified as an armed attack rather than as a mere frontier incident had it been carried out by regular armed forces” (para. 195). By doing so, the Court limits the application of UNGA Resolution 3314, implying that “armed bands, irregulars or mercenaries” need to possess capacities of military strike equivalent to those of regular armed forces.

In its judgment of December 19, 2005, in a case concerning armed activities in the territory of the Democratic Republic of the Congo (DRC) ( Democratic Republic of the Congo v. Uganda ), the ICJ found that the alleged aggression of the DRC against Uganda was not established in law because there was no satisfying evidence of a direct or indirect involvement of the DRC in armed attacks on the Ugandan territory perpetrated by armed groups operating from the Congolese territory (para. 146). According to the Court, these attacks did not emanate from armed bands or irregulars sent by the DRC or on behalf of the DRC, within the sense of article 3(g) of General Assembly resolution 3314 (XXIX) (para. 146). Consequently, the Court found that the legal and factual circ*mstances for the exercise of a right of self-defence by Uganda against the DRC were not present (para. 147). The incapacity of a State to control the activities of groups operating from its territory is not sufficient to establish the act of aggression, since an act of aggression has to be perpetrated by a State or forces acting under the State’s control or on the State’s behalf.

In the case regarding the aggression of Ukraine by the Russian Federation on 24 February 2022, Ukraine asked the ICJ to rule out Russian allegation that its military intervention was legitimate under the 1948 genocide convention to prevent the alleged genocide of Russian minority by Ukraine. In its preliminary order of 16 March 2022, ICJ considered at paragraphs 56 and 57 that the obligation to prevent and punish must be implemented by States in good faith and in accordance with international law, taking into account other parts of the Convention which provide for action within the framework of the United Nations or before the ICJ.

3. International Criminal Court

Originally, the Rome Statute of the ICC, adopted in 1998 and entered into force in 2002, did not provide a definition of the crime of aggression. Because the UNSC was the only competent body to determine the existence of an act of aggression. It did not provide as well conditions under which the Court could exercise jurisdiction over such crime. The competence of the Court to hear cases alleging this crime was then purely theoretical. The Rome Statute provided in article 5 that “the Court shall exercise jurisdiction over the crime of aggression once a provision is adopted in accordance with articles 121 and 123 defining the crime and setting out the conditions under which the Court shall exercise jurisdiction with respect to this crime. Such a provision shall be consistent with the relevant provisions of the Charter of the United Nations.” Articles 121 and 123 of the Statute established the conditions for the amendment and revision of the Statute and notably provided that the Security-General of the UN would convene a Review Conference of the Rome Statute seven years after the Statute’s entry into force. The first Review Conference of the Rome Statute was held from 31 May to 10 June 2010, in Kampala, Uganda. During this conference, the Assembly of State Parties adopted a resolution that included a definition of the crime of aggression and a regime establishing how the Court will exercise its jurisdiction over this crime. The definition of the crime of aggression, proposed by the Special Working Group on the Crime of Aggression, was mainly inspired by the UNGA resolution 3314 of 1974.

The definition of aggression is contained in the new article 8 bis of the Statute and reads as follows:

1. “For the purpose of this Statute, ‘crime of aggression’ means 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.
2. For the purpose of paragraph 1, ‘act of aggression’ means the use of armed force by a State against the sovereignty, territorial integrity or political independence of another State, or in any other manner inconsistent with the Charter of the United Nations. Any of the following acts, regardless of a declaration of war, shall, in accordance with United Nations General Assembly resolution 3314 (XXIX) of 14 December 1974, qualify as an act of aggression:

1. “For the purpose of this Statute, ‘crime of aggression’ means 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.
2. For the purpose of paragraph 1, ‘act of aggression’ means the use of armed force by a State against the sovereignty, territorial integrity or political independence of another State, or in any other manner inconsistent with the Charter of the United Nations. Any of the following acts, regardless of a declaration of war, shall, in accordance with United Nations General Assembly resolution 3314 (XXIX) of 14 December 1974, qualify as an act of aggression:

1. The invasion or attack by the armed forces of a State of the territory of another State, or any military occupation, however temporary, resulting from such invasion or attack, or any annexation by the use of force of the territory of another State or part thereof;
2. Bombardment by the armed forces of a State against the territory of another State or the use of any weapons by a State against the territory of another State;
3. The blockade of the ports or coasts of a State by the armed forces of another State;
4. An attack by the armed forces of a State on the land, sea or air forces, or marine and air fleets of another State;
5. The use of armed forces of one State which are within the territory of another State with the agreement of the receiving State, in contravention of the conditions provided for in the agreement or any extension of their presence in such territory beyond the termination of the agreement;
6. The action of a State in allowing its territory, which it has placed at the disposal of another State, to be used by that other State for perpetrating an act of aggression against a third State;
7. The sending by or on behalf of a State of armed bands, groups, irregulars, or mercenaries, which carry out acts of armed force against another State of such gravity as to amount to the acts listed above, or its substantial involvement therein.”

By the insertion of this new article, the act of aggression became a crime for which individual perpetrators, not merely States, could be held criminally responsible.

The Elements of Crimes of the Court were also amended to specify the constitutive elements of this new crime. One of the elements asserts that the perpetrator must be one or more persons in a position effectively to exercise control over or to direct the political or military action of the State that committed the act of aggression. This definition is restrictive, since it excludes the prosecution of members of non-State armed groups acting on behalf of a foreign State. However, judicial prosecutions against commanders of such groups remain possible before the ICC for other crimes. Besides, recent jurisprudence suggests that attacks by non-State armed groups could be recognized by the judges as constitutive of an act of aggression if it were proven that these groups act as de facto agents of a foreign State.

Unlike other crimes in the Rome Statute, the crime of aggression is subject to several additional conditions of competence. These conditions apply when the referral comes from a State or the Prosecutor’s own initiative (Art. 15 bis ) and not when the referral is made directly by the UNSC (Art. 15 ter ).

The first main difference with other crimes is that the Court cannot exercise its jurisdiction on situations where the aggressor is a State not party to the ICC, even when the State victim of aggression is a State-Party (Art. 15 bis (5)). This departs from conditions set forth in article 12 for the other crimes and constitutes a great limitation to the ICC competence over the crime of aggression. This competence is further restricted since the ICC cannot act when States party involved have not ratified the amendment relating to the crime of aggression (Art. 121(5)) and if it has opted out on this crime according to article 15 bis (4). The second main difference is that the prosecutor may only proceed with an investigation after first ascertaining whether the UNSC has made a determination of the existence of an act of aggression (Art. 15 bis (6)), or when the Pre-Trial Division of the Court has authorized the commencement of an investigation if, six months after the event, the UNSC has not officially determined the existence of the act of aggression (Art. 15 bis (8)).

The competence of the ICC over the crime of aggression entered officially into force on 17 July 2018, when State ratification of the additional conditions established in article 15 bis and article 15 ter (1) have been met by at least thirty States Parties which ratify or accept the amendments and that (2) a decision is taken by two-thirds of States Parties to activate the jurisdiction.

➔ International Criminal Court

4. Organization of American States

The Inter-American Treaty of Reciprocal Assistance, adopted in Rio de Janeiro, Brazil, in 1947, and the Charter of the Organization of American States (OAS), signed in 1948 in Bogota, Colombia, prohibits war of aggression, affirming that victory does not give rights to the attacking State (Art. 3(g) of the Charter). According to the OAS, an act of aggression against one American State is an act of aggression against all the other American States (Art. 3(3) of the Treaty and art. 3(h) of the Charter).

Article 9 of the Treaty defines two types of aggression: (1) unprovoked armed attack by a State against the territory, people, or land, sea, or air forces of another State, and (2) invasion by the armed forces of a State of the territory of an American State. Further, article 21 of the Charter stipulates that “the territory of a State is inviolable; it may not be the object, even temporarily, of military occupation or of other measures of force taken by another State, directly or indirectly, on any grounds whatever.”

Moreover, the OAS recognized the possibility of forms of aggression other than an armed attack (Art. 6 of the Treaty and art. 29 of the Charter). The text does not specify what would constitute this kind of attack, but economic aggression or political subversion and interference could potentially fall into this category.

5. African Union

On 31 January 2005, Member States of the African Union adopted the Non-Aggression and Common Defence Pact in Abuja, Nigeria. This Pact entered into force on 18 December 2009; it was signed by forty-three States but ratified by only twenty-two as of April 2023.

Article 1(c) of this Pact defines aggression more broadly than other international instruments, as it goes beyond the acts committed against the territory of the State and includes attacks perpetrated against the State’s political sovereignty or population.

The Pact also encompasses a broader category of actors who may be held liable for aggressive acts than only those defined by the categories proposed by the UN, the ICJ, and the ICC. Under the Pact, aggression may be perpetrated by armed groups or terrorist groups on the territory of a State (“any foreign or external entity”). Furthermore, any State-sponsored support to armed groups, mercenaries, or other organized transnational criminal groups that may carry out hostile acts against a Member State can constitute aggression. This definition goes much further than the interpretation of “aggression” as defined by the ICJ (see above).

According to the Pact, “Aggression means the use, intentionally and knowingly, of armed force or any other hostile act by a State, a group of States, an organization of States or non-State actor(s) or by any foreign or external entity, against the sovereignty, political independence, territorial integrity and human security of the population of a State Party to this Pact, which are incompatible with the Charter of the United Nations or the Constitutive Act of the African Union.” The following shall constitute acts of aggression, regardless of a declaration of war by a State, group of States, organization of States, or non-State actor(s) or by any foreign entity:

“1. the use of armed forces against the sovereignty, territorial integrity and political independence of a Member State, or any other act inconsistent with the provisions of the Constitutive Act of the African Union and the Charter of the United Nations;

2. the invasion or attack by armed forces against the territory of a Member State, or military occupation, however temporary, resulting from such an invasion or attack, or any annexation by the use of force of the territory of a Member State or part thereof;
3. the bombardment of the territory of a Member State or the use of any weapon against the territory of a Member State;
4. the blockade of the ports, coasts or airspace of a Member State;
5. the attack on the land, sea or air forces, or marine and fleets of a Member State;
6. the use of the armed forces of a Member State which are within the territory of another Member State with the agreement of the latter, in contravention of the conditions provided for in this Pact;
7. the action of a Member State in allowing its territory to be used by another Member State for perpetrating an act of aggression against a third State;
8. the sending by, or on behalf of, a Member State or the provision of any support to armed groups, mercenaries and other organized trans-national criminal groups which may carry out hostile acts against a Member State, of such gravity as to amount to the acts listed above, or its substantial involvement therein;
9. the acts of espionage which could be used for military aggression against a Member State;
10. technological assistance of any kind, intelligence and training to another State for use in committing acts of aggression against another Member State; and
11. the encouragement, support, harbouring or provision of any assistance for the commission of terrorist acts and other violent trans-national organized crimes against a Member State.”

If the African Union determines that a Member State has committed an act of aggression, as defined under the Pact, the Union can put in place mechanisms of collective security (though these mechanisms are still developing). Member States can also submit complaints to the African Court of Justice.

➔ African Union ▸ Collective security ▸ International Court of Justice ▸ International Criminal Court ▸ Public order ▸ Sanctions (diplomatic, economic, or military) ▸ Security Council of the UN ▸ Self-defense ▸ War