Global Affairs November 13, 2023 by

Raffaele Petroni

The principles of self-defence and proportionality in international law: the case of war between Israel and Hamas

The principles of self-defence and proportionality in international law: the case of war between Israel and Hamas

The recent explosion of war between Israel and Hamas following the large-scale attack and massacre perpetrated by the fundamentalist Islamic group in Israeli territory has raised the issue of what constitutes self-defence and proportionality.

Self-defence, in terms of the use of force as a response to an act of aggression and armed attack, is considered to be a fundamental principle in international law (Shaw, 2017). It is primarily crystallised and governed by Article 51 of the United Nations (UN) Charter, which states that “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 main elements that allow a state to invoke the right of self defence and act forcefully to respond to an attack are the following:

  1. Act of aggression and armed attack: Self-defence is only permitted in response to an armed attack. The ethical and legal considerations that allow a state to engage in war are usually referred to as “Jus ad bellum”. Engaging in a war is to be considered as a last resort for a state. These principles are important in international law and ethics to determine the legitimacy of going to war (Bouvier, 2012). 

The definitions of aggression and armed attack have been the subject of much debate in international law. It generally includes actions involving force, such as military actions. In 1974, the UN General Assembly approved a resolution that attempted to define what acts constitute aggression. Article 3 of the UN General Assembly Resolution A/RES/3314 sets a non-exhaustive list (see Article 4) of what constitutes acts of aggression. Despite this attempt, the debate is not settled.

  1. Proportionality and necessity: The ethical and legal rules that govern the conduct of parties involved in an armed conflict are referred to as “Jus in bello”. Self-defence must be proportionate to the threat (not to the damage) and necessary to protect the state. The principles of proportionality and necessity have to be analysed and assessed for each individual operation that is conducted. Excessive force or acts that go beyond what is strictly necessary to the military advantage that can be gained and to repel the attack and threat are not justified. Self-defence is a concept that looks to the future, specifically to the future threats that can be a consequence of the attack. 

In order for the response to be proportional, it has to be aimed at a military target; harm to civilian properties that are not used to conduct armed operations must be minimised as far as possible. The concept of proportionality refers to the means that are employed and the legitimacy of the objectives, the distinction between combatants and non-combatants, the precautions that are taken, and the obligations under International Humanitarian Law (IHL) as expressed in the Geneva Conventions.

The concept of proportionality is also a concept that has been the object of long and harsh debate within the international community. This debate is often fuelled by the number of civilian casualties (often referred to as collateral damages) that are reported.

  1. State responsibility: States are held responsible for their actions in self-defence in terms of international responsibility. Alleged serious violations of IHL precepts and rules may lead to prosecution, both domestically and internationally. 
  2. Pre-emptive self-defence: The concept of pre-emptive self-defence (to be distinguished from the concept of preventive self-defence), where a state takes action against a potential threat before an armed attack has occurred, is a contentious issue particularly in asymmetrical conflicts, where a state faces the threats coming from an insurgent or terror groups, usually referred to as non-state actors. One of the elements that are used to define the extent of control of a territory by a non-state group is the effective control the group has of that territory. States must demonstrate that the threat is imminent and that there are no peaceful alternatives. It’s generally only considered lawful if there’s an imminent threat. In the evaluation of an imminent threat, the elements to consider are the capability to effectively threat, the ability to harm, and the intention to attack.
  3. Collective self-defence: States can also engage in collective self-defence, where one state is attacked, and others come to its aid. This is governed by the principle of collective security, as in the case of NATO and Article 5 of its constitutive instrument.

A situation that has fuelled the debate about the concepts of self-defence and proportionality is the response of the State of Israel to the mass murder perpetrated by Hamas in Israeli territory on October 7th, 2023, whereby its actions, the situation has moved from a state of a ceasefire to a state of open conflict. Particularly intense is the current debate about the high number of civilian casualties among the civilian population of the Gaza Strip as a consequence of the fights between the Israeli Defence Forces (IDF) and Hamas.  

The principle of self-defence applied to this specific situation differs from what someone could generally be accustomed to, which is transitioning from a situation of peace sanctioned by an agreement to a state of war. This scenario does not apply to the situation between the Gaza Strip and Israel. Since the unilateral Israeli disengagement from the Gaza Strip in 2005, there has never been a situation characterised by peace sanctioned by a treaty but alternating phases of high and low tension. The continuous and consistent firing of missiles from the Gaza Strip towards Israeli territory, even during periods of ceasefire like the one that was in place until October 7th, 2023, demonstrates the existence of the armed conflict. Consequently, the Israeli operation should not be evaluated within the framework of the prevention of conflicts (jus contra bellum), nor should it be considered a response to a breach of peace, according to jus ad bellum, but as a dramatic exacerbation of an ongoing armed conflict.

The doctrine related to the principle of self-defence, as prescribed by international law, primarily applies when faced with a “massive attack.” The question would then be what is a “massive attack”. This is a question that does not have an answer “one size fits all”, but it must be evaluated and assessed case by case. The attack perpetrated by Hamas in Israeli territory has changed the conflictual framework that was in place until that moment and brought the hostilities to a higher level and threat. As such, the conflict in itself has then moved from rocket barrages and terror attacks by Hamas counteracted by IDF military operations to a situation where Hamas has proved to be able to conduct complex and organised military operations itself. On these terms, the level of alert for the State of Israel has to move from what Hamas could probably do to what it is theoretically capable of doing. By analogy, this approach will most likely be applied also to other groups similar to Hamas in terms of military capabilities, object and purpose of their own existence and intentions.

Street protests from various parts of the world and social media platforms allege Israel’s disproportionate use of force and commission of war crimes. To determine this, one primarily relies on the severity of images from the Gaza Strip and the reported number of Palestinian casualties. An item of controversy, though is that these figures, provided by the Gaza Strip Health Ministry (ruled by Hamas), do not distinguish between Hamas combatants and non-combatant civilian population. These elements would make these figures unreliable. The decision as to whether a state-employed a disproportionate use of force and/or committed war crimes is a task primarily for judicial courts, whether domestic, international, or hybrid. To this end, certain rules of international law need to be analysed.

The principle of proportionality in the use of force is a general and non-derogable principle of IHL. An allegedly unlawful act only becomes a violation of the law of armed conflicts (LOAC) if it is declared as such by a national or international court. It is not individual states that define violations, nor international organisations (IOs), non-governmental organisations (NGOs), or even the UN Secretary-General. These entities can express opinions and unilateral declarations regarding compliance with humanitarian law, but they cannot establish who has committed war crimes. According to Article 1 common to the Geneva Conventions, “The High Contracting Parties undertake to respect and to ensure respect for the present Convention in all circumstances.” This article is generic in its statement that the parties to the conventions (states) are duty-bound to comply and ensure compliance with IHL from other contracting parties and parties involved in a conflict: they can do so through different means of political pressure, but their statements per se have no legal value, only political significance. Only through the legal principle of universal jurisdiction, with all the difficult implications it can have, states that have not been part of a conflict can prosecute foreign individuals who are suspected of having committed serious violations of IHL within their judicial courts. Up to the moment these states take the political position of moving forward with a legal stance of this type, their statements are to be considered only political statements or invitations.

The criterion of proportionality makes reference to parameters expressed in the jurisprudence of international courts and in doctrine. The hierarchy of priorities in a military operation usually involves 1) achieving the mission, 2) safeguarding soldiers, and 3) protecting the civilian population. Legal analysis focuses on how the precautionary margins of these elements are managed. Proportionality is not assessed by comparing the number of civilian casualties or population affected on both sides of the conflict and the defence systems available. In the case of Israel, this would involve the military apparatus, the Iron Dome anti-missile system, and the widespread availability of anti-rocket shelters in urban centres.  

Rosalyn Higgins, former President of the International Court of Justice (located in The Hague, Netherlands), expressed the thought that the assessment of proportionality “cannot be in relation to any specific prior injury – it has to be in relation to the overall legitimate objective of ending the aggression” (Israel Ministry of Foreign Affairs, 2008; Gold, 2008). This statement implies that there would be a disproportionate use of force if the military force used were employed for purposes other than ending the aggression and threats, causing unnecessary harm to the civilian population. The American jurist Alan Dershowitz clarified the argument by explaining that “proportion must be defined by reference to the threat proposed by an enemy and not by the harm it has produced” (Dershowitz, 2008; Gold, 2008).

Luis Moreno-Ocampo, former Chief Prosecutor of the International Criminal Court, stated that under IHL and the statute of the International Criminal Court “permit belligerents to carry out proportionate attacks against military objectives, even when it is known that some civilian deaths or injuries will occur” (Office of the Prosecutor, International Criminal Court, The Hague, February 9, 2008; Gold, 2008). The death of civilians during an armed conflict, however grave and regrettable, in itself does not constitute a war crime. It would become a war crime if an army purposely targeted civilians or acted with negligence or recklessness. Moreno-Ocampo’s opinion mentions military objectives defined by Article 52.2 of Protocol I on the Protection of Victims of International Armed Conflicts (Geneva, June 8, 1977) – an additional protocol to the Geneva Conventions of 1949 – as follows: “Attacks shall be limited strictly to military objectives. In so far as objects are concerned, military objectives are limited to those objects which by their nature, location, purpose or use make an effective contribution to military action and whose total or partial destruction, capture or neutralization, in the circumstances ruling at the time, offers a definite military advantage.

According to these opinions, the manner in which the operation is conducted, the means used in relation to the objective, the available alternatives, and the precautions taken by the army in question to prevent the killing of non-combatant civilians must be evaluated and taken into consideration. In this sense, notifications sent to the population via SMS, “knock-on-the-roof” warnings, and flyers indicating where to find shelter or evacuate are means used by the IDF to inform the civilian population in the Gaza Strip and reduce the margin of danger for the civilians themselves.

The war between Hamas and Israel has also raised questions about the involvement of hospitals and other medical facilities in the fighting. In terms of IHL, the law that rules over this issue is found in articles 18 and 19 of the Fourth Geneva Convention. Article 18 concerns the protection of hospitals and states that “Civilian hospitals organized to give care to the wounded and sick, the infirm and maternity cases, may in no circumstances be the object of attack, but shall at all times be respected and protected by the Parties to the conflict. 

States which are Parties to a conflict shall provide all civilian hospitals with certificates showing that they are civilian hospitals and that the buildings which they occupy are not used for any purpose which would deprive these hospitals of protection in accordance with Article 19 .

Civilian hospitals shall be marked by means of the emblem provided for in Article 38 of the Geneva Convention for the Amelioration of the Condition of the Wounded and Sick in Armed Forces in the Field of August 12, 1949, but only if so authorized by the State.

The Parties to the conflict shall, in so far as military considerations permit, take the necessary steps to make the distinctive emblems indicating civilian hospitals clearly visible to the enemy land, air and naval forces in order to obviate the possibility of any hostile action.

In view of the dangers to which hospitals may be exposed by being close to military objectives, it is recommended that such hospitals be situated as far as possible from such objectives.

Article 19, instead, rules over the discontinuation of the protected status expressed in Article 18 by stating that “The protection to which civilian hospitals are entitled shall not cease unless they are used to commit, outside their humanitarian duties, acts harmful to the enemy. Protection may, however, cease only after due warning has been given, naming, in all appropriate cases, a reasonable time limit, and after such warning has remained unheeded.

The fact that sick or wounded members of the armed forces are nursed in these hospitals, or the presence of small arms and ammunition taken from such combatants and not yet handed to the proper service, shall not be considered to be acts harmful to the enemy.

A further element of controversy is the distinction between combatants and non-combatants. While the IDF has clear and distinguishable uniforms, the fact that Hamas does not use distinguishable uniforms for its militants increases the risks of non-combatants being mistaken for combatants. Similarly, the use of civilian infrastructures to hide and attack leads to the turn of these infrastructures from civilian use to military use, therefore becoming legitimate targets.

In conclusion, the debate about self-defence and proportionality is a debate that does not have a settled answer. These two principles must be carefully considered and assessed neither generically nor on the grounds of the damage and civilian casualties but on a one-by-one basis, taking into account the elements that brought to the decision to target a specific infrastructure. 


  • United Nations Charter
  • Geneva Conventions
  • UN General Assembly Resolution A/RES/3314
  • The North Atlantic Treaty (NATO), 1949
  • Bouvier A., International Humanitarian Law and the Law of Armed Conflict, Peace Operations Training Institute, 2012
  • Bryen S., Proportionality: Israel’s Response to Hamas Rocket Attacks under International Law, The Jewish Institute for National Security Affairs (JINSA), January 10, 2009
  • Dershowitz A., “The Hamas Government Has Declared War Against Israel: How Should Israel Respond?” Huffington Post, March 14, 2008
  • Gold D., Did Israel Use ‘Disproportionate Force’ in Gaza?, Jerusalem Center for Public Affairs (JCPA), December 28, 2008
  • Israel Ministry of Foreign Affairs, “Responding to Hamas Attacks from Gaza – Issues of Proportionality Background Paper,” March 2008
  • Office of the Prosecutor, International Criminal Court, The Hague, February 9, 2008
  • Shaw M., International Law, Eight Edition, 2017

About the author

Raffaele Petroni

Raffaele Petroni