1. Introduction

Since the drafting of the United Nations Charter in 1945, the concept of self-defense had played a crucial role in the international legal framework. UN Charter Article 51 allows states to self-defense against an armed attack until the United Nations Security Council acts to restore international peace and security. However, the Council’s measures are often limited, in terms of both the number of distinct approaches and their effectiveness. Besides, the unclear definition of an “armed attack” is also an important factor leading to wrongful interpretations of the Charter. This seemingly minor problem has been the root cause of multiple armed conflicts observed in the past and is continuing to facilitate more in the present and the near future.

Scholars had attempted to define Article 51 and relevant terms under it, such as Tom Ruys’ discussion of armed attack in his book ‘Armed Attack’ and Article 51 of the UN Charter [1], and to what extent the use of force is being permitted, but there was never a commonly recognized answer. Some consider the role of customary international law in their definition, while some directly challenge the Purposes of the United Nations itself [2,3]. Regardless of their validity, each scholar interpretation does provide valuable insight and allows us to proceed one step further in finding the key to unlocking the ideas behind the rules governing the use of force.

2. Thesis and Road Map

The research question of this paper is whether the rules governing countries’ right to self-defense are too permissive of unjustified acts of aggression against other countries and what should be done to narrow them. The thesis of this paper is that countries often justify illegal uses of force through Article 51's allowance of self-defense, suggesting that the terms under the Article are too vague and provide legal loopholes. International courts also fail to adequately adjudicate these aggression-related issues. These suggest that clearer guidelines on relevant rules and more rigorous means of enforcement is demanded by the present international legal system.

The next part of the paper presents scholarly viewpoints towards the rules regulating the use of force under international law and reveals limitations of existing viewpoints. Part 4 of the paper discusses the methodologies conducted in this research and outlines a number of hypotheses. Part 5 of the paper evaluates past and present conflicts that show practical applications of rules regulating the use of force. Part 6 provides recommendations towards the research question based on evaluation and findings. Part 8 concludes the paper and its outcomes.

3. Literature Review

3.1. UN Charter Article 2(4) and its Exceptions

The use of force of any UN Member State is prohibited by Article 2(4) of the UN Charter. It forbids the threat and/or actual application of force by member states in their international relations. There are, however, two written exceptions to this general regulation on using force, which are (i) UN Security Council authorization and (ii) the inherent right of self-defense of each country under UN Charter Article 51. Article 51 clearly indicates that not even the Charter itself shall harm the inherent right of states' self-defense, which undoubtedly includes Article 2(4).

Based on these articles followed interpretations on the different uses of force and if they are deemed lawful under the regulations. Haque [4], after going through a series of evaluation upon the drafting process of the UN Charter, concludes several acts of self-defense consistent and inconsistent with the Articles and the purposes of the UN: (1) Since only the Security Council has the authority to use force to stop and eliminate a threat to the peace, self-defense is not allowed prior to an armed attack. (2) Because only the Security Council has the authority to use force to put an end to a breach of the peace, self-defense is not allowed in reaction to a use of force that is not severe enough to be considered an armed attack. (3) In the event that a non-State actor uses violence on the territory of a non-consenting State, States involved are required to resolve their conflicts peacefully and, if necessary, seek the Security Council's supervision before engaging in self-defense [4].

3.2. Anticipatory Self-Defense

Besides from the two written exceptions to Article 2(4), it is also important to know that there exists a doctrine named after the American steamboat “the Caroline.” The Caroline doctrine originated from the United Kingdom forces' 1837 demolition of the Caroline in effort to stop the United States from supporting Canadian revolutionaries, during which Canada was a colony of the UK. The UK authorities believed that the Caroline posed a threat to it, while those of the US argued that the UK could have settled the dispute by peaceful means. Letters [2] were exchanged between the two states, and it can be inferred from the letters that they have come to an agreement at some extent regarding their common recognition on certain customary rules.

Many scholars like Matthew Waxman [5] believed that their agreement could be understood as a customary international law regulating anticipatory or preemptive self-defense, which is basically when countries can use force against another country in advance of an armed attack that is “instant, overwhelming, and leaving no choice of means or moment of deliberation,” according to Daniel Webster’s claim in a letter given to the UK ambassador. Noting that a customary international law exists in conflict to those under the UN Charter, Occelli [2] challenges this view in her paper, mentioning a number of points. She provided a detailed account of the factual events behind the Caroline incident that casts doubt on the assertion that letters exchanged between the US and the UK back then were an agreement on a new customary international law relating to self-defense, and claims that no actual acknowledgement under international law was formed. She cited the ruling by the International Court of Justice (ICJ) holding that adoption of certain rules by states alone is insufficient for the Court to treat them as part of customary international law. She concludes that their “agreement” was no more than diplomatic acts to avoid war and thus does not intend to contribute to advancements of customary law [2].

3.3. Interpretations of the Charter

In the past few years, rules governing countries’ rights to self-defense preemptively against potential threats are being used as justifications for states’ armed attack against other states. Countries like Russia are justifying their armed attacks on political rivals using self-defense under Article 51. In attempt to explain this phenomenon, Hurd [3] provides a different explanation of the UN Charter’s prohibition of war between states. He argues that the Charter “encourages states to go to war under the banner of self-defense.” He mentions that one of the aims of the Charter was to change practices of states towards war through a definition of justified war acts, and this was also the reason for the encoding of self-defense as an inherent legal right [3]. However, Hurd [3] fails to connect his justification of war with an important element of the UN: its purpose to promote international peace and security. Obviously, an act of war is a clear breach of this purpose, and thus can never be justified under the UN Charter. The allowance of lawful use of force should not be confused with going to war as they are distinct concepts with different scales of effect.

4. Methodology and Hypotheses

In order to investigate the research question, this research will be conducting a qualitative approach, reviewing primary and secondary sources. Primary sources include Security Council resolutions and judicial decisions from the International Court of Justice or the International Criminal Court. Secondary sources may include academic articles and expert commentaries. This research will also include comparisons of interpretations on legal frameworks from different scholars. In addition, case studies will also be involved to explore specific circumstances in which the application of international law may be distinct, such as the Russo-Ukrainian conflict and the Iran-Israel conflict. Sources will derive from internet platforms including Google Scholar and the Social Science Research Network (SSRN), and news article columns. A minimal amount of data is anticipated due to the particularity of the research question, so quantitative approaches will not be prioritized.

This paper tests five hypotheses as potential answers to the research question. They are:

(1) The rules governing countries’ rights to self-defense are too permissive due to the ambiguity in Article 51's definition of self-defense.

(2) The rules governing countries’ rights to self-defense are too permissive because the development of customary international law expanded the boundaries of justifiable self-defense.

(3) The rules governing countries’ rights to self-defense are too permissive because the Security Council’s response mechanisms allow state exploitation of self-defense provisions for aggressive purposes.

(4) The rules governing countries’ rights to self-defense are too permissive because International Courts are not addressing/prosecuting cases promptly.

(5) The rules governing countries’ rights to self-defense are not too permissive, and the prevailing of armed conflict is caused by disobedience of states.

5. Case Studies

This part of the paper analyzes real-world conflicts that were justified as self-defense under UN Charter Article 51, each under unique circumstances and conditions. Through analysis, they could provide insight on the international community’s attitude (mainly institutions like the UN and international courts) towards severe conflicts.

5.1. Russo-Ukrainian War

The Russian Federation launched an armed invasion on Ukrainian territory on 24 February 2022, causing severe levels of casualty. Having recognized Donetsk and Luhansk as republican states, Putin justifies his use of military force on Ukraine with “collective self-defense” under Article 51 of the United Nations Charter [6] in accordance with his statement that Ukraine was “committing genocide against Russians in the territory.” However, there was no evidence that Ukraine engaged in actions defined by the Genocide Convention against any particular group in eastern Ukraine [7].

The United Nations Security Council held a meeting on 25 February 2022 to address the Ukraine crisis, but fails to adopt a Draft Resolution due to the Russian Federation’s veto [8]. This eliminates the possibility of Security Council action to reestablish global peace and security in regards to the Ukrainian conflict.

The Russo-Ukrainian war is a recent example of a state using self-defense to justify its attack on another country. The fact that Russia justified its act of force against another nation with Article 51 despite being illegal suggests that the vagueness of Article 51 left room for states to interpret and legalize their use of force.

It is also important to note the Security Council's and the International Criminal Court's obligated role in this case. The SC was supposed to pass a resolution that addresses the ongoing conflict, but all resolutions proposed were vetoed by Russia, implying that the current Security Council mechanism overly permits the use of force for its permanent members. Meanwhile, despite that the ICC did issue an arrest warrant, neither Russia nor other countries were cooperative in capturing Vladimir Putin for his prosecution at the ICC, reflecting the ineffectiveness of the Court without national assistance.

5.2. Iran-Israel Conflict

The Israel-Hamas war began on 7 October 2023, when the Hamas initiated an attack against Israeli civilians [9], during which Iran supported the Hamas. In retaliation, Israel launched an airstrike on the Iranian consulate located in Damascus, the capital of Syria, on 1 April 2024, killing several senior military commanders [10]. Iran responded by launching drone and missile attacks on Israel. By the end of April, after rounds of firepower exchanges, both sides show intents to deescalate the ongoing conflict.

Alarmed by the devastating event, UN Secretary-General António Guterres condemns Iran’s missile and drone attacks in retaliation against Israel and “calls for an immediate cessation” of hostilities [11]. UN experts and the Office of the High Commissioner for Human Rights (OHCHR) criticized both states’ actions and emphasized that such acts are violations of UN Charter Articles 2(4) and 51 [12]. They expressed their hope that the United Nations take immediate action to prevent further conflict. However, the Security Council has not yet taken any approach or drafted any resolutions in addressing the Iran-Israel conflict.

Although the International Court of Justice ruled that Israel halt ongoing military actions in Gaza to address the Israel-Palestine conflict [12], it made no comments on the conflict between Israel and Iran. On the contrary, The International Criminal Court prosecutor Karim Khan sought to issue arrest warrants on holding leaders of Israel and Hamas responsible for possible war crimes [13], but he has yet to accomplish any further actions.

Unlike how international institutions had taken prompt actions to mitigate the Russo-Ukrainian conflict, there had not been many practical measures done to tackle the ongoing tension between the states of Iran and Israel. This suggests that the political positions of the countries involved or the conflict’s state of being insufficiently grave may be some of the factors of its failure to evoke reactions as strong as those against the war between Russia and Ukraine from the international community.

A questionable factor on whether Israel’s strikes can be justified as self-defense would be the proportionality and necessity under the circumstances. It is doubtful whether the elimination of several militants is proportional to the death of about 1,000 citizens [9] and requires a judgement from international courts. Secondly, many of Israel’s armed attacks were based on the assumption that Iran’s development of nuclear weapons will pose a threat to Israel’s future presence. However, no evidence proved that Iran actually undertook and is still continuing its development of weapons of mass destruction, making Israel’s justification of its act more similar to how preemptive self-defense is widely believed to work under customary international law.

The case shows countries taking violent measures against a use of force on itself, and whether it constitutes self-defense. It shows another example of wrongful interpretation of the law possibly due to vagueness of its definition. Future jurisdictions by international courts can help determine whether force application in such circumstances is deemed acceptable by the international community.

5.3. War in Afghanistan

In the year of 2001, president George Bush demanded that the Taliban immediately bring Osama bin Laden, a suspect of the September 11 attack, to the US. The Taliban refused because no direct evidence proves his involvement in the attack. On October 2001, US forces invaded Afghanistan under the Taliban control and ousted the Taliban leadership [14]. The Taliban relocated to Pakistan, while the US troops remained in Afghanistan to work on reforming its government and counter-terrorism.

Back in 15 October 1999, having observed the uprising of al-Qaeda, the United Nations Security Council adopted Resolution 1267 and established a committee to monitor the al-Qaeda and Taliban to prevent further acts threatening international peace and security [15]. It also imposes sanctions on its transportation and shipments.

The US justifies its military actions since 9/11 with self-defense under UN Charter Article 51, stating that the Taliban committed aggression against it [16]. The involvement of other countries such as the United Kingdom and Canada in the Afghanistan operation were justified under Article 5 of the North Atlantic Treaty and collective self-defense under the UN Charter [17].

In its 1999 and 2001 resolutions, the Security Council did not authorize the use of force in its resolutions under Article 42 against the Taliban, but affirmed the act of self-defense under the circumstance and condemned terrorist acts [15,18). It expressed its concern and reached out to the international community to suppress terrorism after observing the incident.

We can see how Article 42 is applicable on non-state actors, how the UN acknowledges their potential in being a serious threat to international security, and how the international community supports the use of force against terrorists. However, this case was exceptional because it involved the attempt of one country to reform another country’s government system in order to prevent future threats. This at some extent is relatable to anticipatory self-defense, in which countries takes action before an actual offense happens, but should also be considered carefully if the interference of a country’s government is allowed by another country just because it claims to eliminate a potential threat.

5.4. Turkish Involvement in Syrian Civil War

Turkey had a significant refugee inflow as a result of the Syrian crisis. Gradually, the country can hold no more refuge as it consumes too much of its resources, so Turkey decided to interfere with the conflict and establish a “safe zone” in Syria for refugee settlement [19]. Turkey's engagement in the Syrian Civil War has evolved since 5 December, 2011, from military assistance to direct military interventions [20]. By 2020, Turkey had already killed thousands of Syrian troops and destroyed several military aircraft and tanks [21].

Turkey justified its invasion on northern Syria with self-defense under UN Charter Article 51 to counter an “imminent terrorist threat [22].” It promises the United Nations that “the operation will only target terrorists and their hideouts [19].”

The United Nations sees this as a violation of the UN Charter. The Security Council drafted two resolutions concerning the situation in Syria, with both resolutions failing to pass [23]; one called a ceasefire but was vetoed by Russia and China, while the other supported counter-terrorism approaches but did not pass.

Since neither Turkey nor Syria is a member of the Rome Statute, the International Criminal Court has not entered into investigations towards the issue. The International Court of Justice, on the other hand, has not received cases brought by other countries that directly addresses the conflict, and thus has done nothing to mitigate the issue.

This case shows yet another instance of a wrongful interpretation of Article 51. The SC fails to propose resolutions because of its veto mechanism. International Courts were also proven ineffective and inadequate when addressing issues that exist but were not brought to court, implying that the courts are either unwilling or unable to put more of its attention to crises around the world.

5.5. Cross-Case Analysis

Some of the cases studied show extents of generality. For instance, both Cases (iii) and (iv) involved non-state actors, particularly terrorists, that are not taken into much account of by the international legal system. These actors could pose grave threats to international peace and security without necessarily being linked to a particular state, but whether the current legal rules apply to them still remains disputable.

Beyond that, the actors of Cases (i) and (ii) perceived anticipatory self-defense as appropriate, despite it not being defined under the Charter and the Caroline doctrine being controversial. Although this particular act was deemed illegal by the ICC and the Security Council in the Russia-Ukraine case, it does not serve as an excuse to not explicitly prohibit it under the Charter.

Finally, international institutions are proven ineffective in addressing armed conflicts both before and after they occur. Regardless of their response, existing measures pose no deterrence on preventing such acts and have limited effect on halting ongoing crises.

6. Recommendations

After going through rigorous analyses, it can be concluded that there exists an overly permissive situation of using force due to countries’ heavy reliance on the vaguely defined act of self-defense under Article 51 and international courts failing to adequately adjudicate these issues. An appropriate level of permissibility for self-defense should balance the right of states to protect themselves while preventing excessive or unjustified use of force. This paper hereby proposes the following recommendations to ameliorate the status quo of the overly permissive use of force:

6.1. Legal Recommendations

States, including those of great power, are encouraged to draft a new Treaty that stipulates the use of self-defense in precisely defined terms. Based on the outcomes of the Case Studies, it is believed that there exists a need to define the applicability of self-defense under certain conditions, such as the involvement of Non-State Actors. The author believes that states are fundamentally peace-loving, so an incentive of drafting the treaty would be global peace, something previously broken due to existing weaknesses of the UN Charter. Given that amending the Charter itself would be challenging, this is an effective way that aligns with the “free and voluntary” principle of the international legal framework. If it would be too difficult for states to directly come up with a global-scale treaty, they can begin with regional treaties before slowly approaching a universal treaty.

Besides from treaty law, states are also recommended to discuss about the application of self-defense as customary international law. Though the Caroline doctrine exists, its applicability remains controversial, and a new custom could better promote state cooperation and legal responsibility. It is recognized that customary law lacks binding force and enforcement mechanisms when compared to treaty law, so states should make use of international forums like the United Nations General Assembly and the United Nations Security Council to promote active discussion and supervision of compliance. Future determination of effectiveness could derive from Opinio Juris, when states naturally believe that they are obligated to comply with such rules as part of customary law.

6.2. International Criminal Court Empowerment

A direct way of expanding the ICC’s jurisdiction would be letting more countries enter the Rome Statute. Many countries currently in conflict with another are not part of the Rome Statute, and the ICC does not hold jurisdiction against their cases. Many states, especially those currently engaging in conflicts, may see ICC jurisdiction as a threat to their sovereignty. Therefore, it is of utmost importance to strengthen the neutrality of the Court through transparent investigation processes.

Alternatively, the ICC could be funded by governments and the UN to improve its capability of prosecuting several cases at the same time. One of the ICC’s existing limitations include it being unable to handle multiple cases at a time. With sufficient funding, it will be more willing and able to actively investigate cases happening around the world.

With funds provided, its prosecutor Mr. Karim Khan should also take a more aggressive approach in exercising the court’s jurisdictions. It is of his duty to ensure that the Court is operating at its full capacity and make the most out of it to contribute to maintaining international peace and security while prioritizing high-impact cases to set precedents for future prosecutions.

7. Conclusion

The cases studied confirm that states often justify their use of force using self-defense under UN Charter Article 51, highlighting its vague scope and the resulting misinterpretations. The second hypothesis was irrelevant to the cases studied and is not considered further. The third hypothesis was supported, as the Security Council's actions were insufficient to stop the conflicts, with resolutions having minimal impact on issues due to various political factors. The fourth hypothesis was also supported, having observed the slow responses and limited engagement from the International Criminal Court and International Court of Justice. Lastly, the fifth hypothesis was incorrect, as the vagueness of Article 51 makes it inappropriate to assume that states are willfully breaching it.

The legal loopholes deriving from the vagueness of self-defense undermines the purposes of the United Nations to maintain international peace and security. If the rightfulness of state acts cannot be determined, regulations would be pointless. In order to address this challenge, state cooperation and enforcement of the law is crucial. Through the implementations of the recommendations proposed above, the international society can unveil new possibilities to the international legal system and promote a more stable and predictable legal order.

While this paper addresses some of the key aspects of the issue, it alone will not be able to fully eradicate the problem. The collective effort of scholars to conduct further research is both necessary and encouraged to develop more effective solutions.