Abstract
Article 51 of the United Nations Charter enshrines the inherent right of states to self-defence in response to an armed attack. However, its traditional framework, designed for inter-state warfare, has been increasingly challenged by the rise of asymmetric threats posed by non-state actors such as terrorist groups and insurgent militias. This paper critically examines how the application of Article 51 has evolved—sometimes dangerously—beyond its original scope, especially when invoked to justify cross-border military actions against non-state actors operating from the territory of other sovereign states.Through a detailed analysis of state practices and case studies—including Israel’s operations in Gaza and Lebanon, Turkey’s repeated incursions into Syria and Iraq, and the U.S.-led coalition strikes in Syria—this research highlights the legal, ethical, and humanitarian consequences of expanding the doctrine of self-defence. It explores contentious developments such as the “unwilling or unable” doctrine, the lack of a defined threshold for “armed attack,” and the growing reliance on anticipatory self-defence, all of which contribute to a troubling erosion of core principles like sovereignty, necessity, and proportionality.The paper identifies key gaps within Article 51 that have enabled states to justify unilateral force without proper accountability or Security Council oversight. It concludes with a set of recommendations aimed at clarifying the legal thresholds, enhancing procedural safeguards, and integrating customary international principles to ensure the lawful and restrained use of force. Ultimately, this study argues that Article 51 must evolve to address the realities of modern conflict without undermining the foundational norms of international law it was designed to uphold. Introduction:
In an age where wars are being waged not only by armies but by networks, militias and ideology, the traditional right of self-defence is being stretched to their breaking point. After the global massacre of what was the devastating World War II, the world leaders advocated for lasting world peace and the United Nations was born through the establishment of UN Charter. Since then, many years has passed and this international organization has tackled many pressing issues over the years. Global tension has risen exponentially ever since the arms race kicked off between the first world countries. As a result,clashes and armed conflicts also has risen since the second world war.There has been a total of 285 distinct armed conflicts since World War II.[1] The formation of armed conflicts has also changed drastically. The foundational framework of international law governing the use of force, according to Article 2(4) and the exception of it, Article 51 of the United Nations Charter, was crafted in an era when armed conflict was largely conceived as a matter between sovereign states. However, the emergence and proliferation of non-state armed groups—ranging from transnational terrorist organizations to insurgent militias—have significantly disrupted this traditional paradigm. In recent decades, countries have increasingly applied military force across borders in the name of self-defence against non-state actors operating from the territory of other sovereign states, often without the latter’s consent. Moreover, there has been a total disregard to laws of armed conflict as significant civilian casualties have been a common by product of these armed conflicts in the name of self-defence.This shift has generated intense legal controversy. While some argue that customary international law has evolved to accommodate such practices, others maintain that these actions violate core principles of state sovereignty, non-intervention, and collective security. The invocation of doctrines such as “unwilling or unable” to justify these uses of force further complicates the legal landscape, raising fundamental questions about the boundaries of self-defence and the risk of unilateralism under the facade of security.
This paper examines whether and to what extent the use of force is permitted under international law in self-defence against non-state actors. It critically analyses the emerging state practices, while applying qualitative research methodology with relevant jurisprudence, and doctrinal developments, with a view to determining whether current interpretations of Article 51 represent a necessary evolution or a dangerous expansion. While exploring such dimensions, this paper will dive into contemporary instances such as Israel’s war with Hezbollah and Hamas, Turkey’s operations against Kurdish groups in Syria and Iraq; finallythe armed attack by the coalition of US, UK and Franceagainst ISIS on Syrian territory.By exploring the legal, ethical, and practical dimensions of this issue, the study aims to clarify whether international law is keeping pace with the realities of modern conflict—or merely being stretched to accommodate political convenience.
1. Traditional Understanding of Article 51:
Since the inception of human society, war has been seen as an inevitable part of international politics. However, Schrijver has stated that, “unregulated warfare deteriorated into cruelties, as the First and Second World Wars proved to be too cruel and too bloody for soldiers and indiscriminate for civilians.”[2] As a response, the world leaders came together and formulated the UN Charter in 1945, in which Article 51 stated,
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. Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security.[3]
Through this provision, the UN has recognized an inherent right of individuals and state actors to defend in the event of such an attack, which marks the sole explicit exception to the principle prohibiting use of force under Article 2(4) of the UN Charter.[4] Traditionally, an “armed attack” referred to an attack or use of force orchestrated by a state against another state.[5]But there are distinctions on the term “use of force” as well. The ICJ in the Nicaragua case, distinguished between “the most grave forms of the use of force (those constituting an armed attack) from other less grave forms,” requiring a certain “scale and effect” to trigger Article 51.[6]Apart from these aspects, certain customary international principles have always had their place in Article 51, although not explicitly stated. These are the principles of necessity and proportionality. Working as complimentary to Article 51, states have always maintained these principles while applying Article 51 to ensure lawful exercise of self-defence. A landmark example can be found in the Caroline incident.The then US Secretary of State, Webster, was responding to a British claim that it had a legal right to conduct an attack within American watersagainst an American Ship called the Caroline, because it contained armed men who intended to support the Canadian Rebellion of 1837.[7]In response Webster argued that, “to admit that claim Britain must show a necessity of self-defence, instant, overwhelming leaving no choice of means and no moment for deliberation.”[8] This exercise of necessity is later reflected and upheld in another dispute called the 1956 Suez Crisis, where Derek Bowett stated customary international law grants the use of armed force in situations of ‘necessity’.[9] But since then, the practice of this provision has gone through significant evolution over the years.
2. Expansion Beyond Article 51 and Rise of the Non-state Actors:
Ever since the Caroline incident, the scope of the meaning of Article 51 has gone through some rigorous expansion. Just like the Suez Crisis of 1956, there are other instances where state members have stretched the meaning of Article 51 to new limits in order to gain political advantage over rival countries. For example, during the cold war era, the USSR and the USA have disregarded the principles of international law in many instances while the USSR focused on spreading Soviet influence, and the USA resisted communist expansion.[10] In another instance, the US invoked Article 51 when they attacked Libya and Iraq after accusing them of trying to kill US nationals despite the targets having official status. In 1986, the USA conductedair strikes in Tripoli and justified it as self-defence in response to past terrorist attacks by Libyans against US nationals, as well as to deter future terrorist action.[11]Again in 1993, the USA tried tojustified its missile attack at the Iraqi intelligence headquarters located in Baghdad, which was a response to an assassination attempt on the former US President Bush, carried out in Kuwait two months before.[12] A rather controversial interpretation of Article 51 came from the United Kingdom, when they tried to reclaim the Falklands islands under the mentioned provision. The UK applied a rather unique interpretation of Article 51 where they found that the intention behind Article 51 was to allow the use of force “in the heat of the moment”.[13] These were just states trying to justify their actions to gain advantage in international power politics, but a different shift came altogether in 2001, after the 9/11 attack in the USA. It is when the first time in the definition of “armed attack” terrorist attack got included.[14] Subsequently, the UN Security Council responded through resolution 1368 and 1373 which respectively affirmed the right of the member states to respond to terrorist attacks and forbade funding/aiding of terrorist groups.[15] This was one of the first instances where a non-state actor came into the ambit of the scope of Article 51. A global war on terror kickstarted and as a result the US invaded two countries back-to-back, Afghanistan and Iraq. The invasion of Afghanistan was deemed as self-defence and war against terror which established a very broad interpretation of Article 51.[16] In the case of Iraq, the justification of the invasion was mainly that Iraq was a threat to the national interest of the US and as a result the US invoked Article 51.[17] Since then, both the US and the UK has tried to establish through various means that military force against non-state aggressors like the Al-Qaeda is permissible under Article 51.[18] The US has since then developed the “unwilling or unable” doctrine as a justification for unilateral intervention, which basically elaborates that self-defence can be invoked when a host state is considered to be “reluctant or incapable” to deal with the non-state actors who are conducting armed attacks from their territory.[19] Although this principle has been endorsed by prominent figures in global politics like the US, countries like Mexico and Brazil has rejected this principle for its vagueness and for the fear of being misused in a broader context.[20] However, in modern times we can see the increasing flexibility of Article 51, like the Turkey’s incursion on Syria in 2019 invoking Article 51 and citing “imminent terrorist threat”[21]; or, the 2022 attack on Ukraine by Russia where Russia pursued their right to “collective self-defence ” for eastern Ukrainian regions[22]. All of the more recent incidents are heavily debated and it has increasingly comeinto focus that the interpretation of Article 51 might have been stretched too far by some countries in order to justify their actions.
[1] ‘Major Military Operations Since World War II | Infoplease’ <https://www.infoplease.com/history/us/major-military-operations-since-world-war-ii> accessed 27 July 2025.
[2] Shakespear Hamauswa, ‘A Critique of United States’ Application of Article 51 of the United Nations Charter in Iraq and Afghanistan’ 219.
[3] United Nations, ‘United Nations Charter (Full Text)’ (United Nations) <https://www.un.org/en/about-us/un-charter/full-text> accessed 27 July 2025.
[4] Jan Arno Hessbruegge, Human Rights and Personal Self-Defense in International Law (Oxford University Press 2017) 2 <https://academic.oup.com/book/25556> accessed 27 July 2025.
[5] Mary Ellen O’Connell, Christian J Tams and Dire Tladi, Self-Defence against Non-State Actors (1st edn, Cambridge University Press 2019) 55 <https://www.cambridge.org/core/product/identifier/9781108120173/type/book> accessed 27 July 2025.
[6] Andrew Clapham, ‘The Use of Force after the UN Charter’ in Andrew Clapham, War (1st edn, Oxford University Press 2021) 125 <https://academic.oup.com/book/57867/chapter/471842578> accessed 27 July 2025.
[7] Hamauswa (n 2) 223.
[8] ibid
[9] O’Connell, Tams and Tladi (n 5) 214.
[10] Hamauswa (n 2) 222.
[11] ‘A Critical Study of Legitimization of Preemptive Self-Defense as a Counter-Terrorism Measure Under International Law – WMO’ (16 November 2023) <https://worldmediation.org/a-critical-study-of-legitimization-of-preemptive-self-defense-as-a-counter-terrorism-measure-under-international-law/> accessed 28 July 2025.
[12] ibid
[13] Mulwa Mwende Valentine, ‘Re-Assessing the Right to Self Defence in International Law: Revisiting Article 51 of the Charter of the United Nations’ 100.
[14] Hamauswa (n 2) 223.
[15] ‘A Critical Study of Legitimization of Preemptive Self-Defense as a Counter-Terrorism Measure Under International Law – WMO’ (15 April 2020) <https://worldmediation.org/a-critical-study-of-legitimization-of-preemptive-self-defense-as-a-counter-terrorism-measure-under-international-law/> accessed 28 July 2025.
[16] Valentine (n 13) 204.
[17] Hamauswa (n 2) 230.
[18] Oona A Hathaway, ‘How the Expansion of “Self-Defense” Has Undermined Constraints on the Use of Force’ (Just Security, 18 September 2023) <https://www.justsecurity.org/88346/the-expansion-of-self-defense/> accessed 28 July 2025.
[19] Carina Gouvêa, ‘Unilateral Humanitarian Interventions and the Legitimate Use of Force: Article 51 of the United Nations Charter and North American Interpretations’ [2020] SSRN Electronic Journal 7 <https://www.ssrn.com/abstract=3679241> accessed 28 July 2025.
[20] Clapham (n 6) 79.
[21] Hathaway (n 18).
[22] ibid.