Applications of the UN Charter’s Article 51: The Use of Self-Defense in Outer Space
In 2022, Russia invaded Ukraine on the premise of protecting its citizens abroad. In particular, President Putin justified his aggression against Ukraine by claiming that those residing in Ukraine’s Donetsk and Luhansk regions desperately required aid, resulting in his invocation of Article 51 of the United Nations (UN) Charter. [1] The use of force in international politics is a contested factor that remains relevant and predominant throughout discussions of global security. This is primarily because the UN Charter’s strict ban on the use of force is contradicted by Article 51, which affirms the right to self-defense. Article 51 dictates, “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….” [2]
This right, afforded to member States, effectively allows for an exception to the prohibition on the use of force. However, this extensive power relies on an especially ambiguous term: armed attack. In arenas where the UN lacks effective regulatory measures, such as outer space, Article 51’s vagueness remains a critical threat to international security. Article 51 is easily exploited due to its use of the undefined term, armed attack, and remains a precarious loophole to the prohibition on the use of force. The vagueness surrounding the standard for invoking Article 51 risks the security of international relations. Today, this danger is clearly illustrated by President Putin’s reliance on Article 51 to justify his aggression. Currently, Russia’s veto power prevents the UN from effectively imposing sanctions, exposing a crucial weakness in the UN’s governing power. [3] Although the UN has demonstrated its punitive powers through condemnations against Russia, this critical lacuna is only heightened in international law regarding the regulation of outer space, a sector where no state, including the UN, may declare sovereignty. [4] Thus, the UN has a significantly limited capacity to prevent a potential exploitation of Article 51, a vulnerability plainly exposed through an analysis of existing legislation for outer space.
Outer space is predominantly regulated by five treaties, including the Treaty Governing Activities of States in the Exploration and Use of Outer Space Including the Moon and Other Celestial Bodies (Outer Space Treaty), which maintains a ban on nuclear weapons in space and emphasizes the nature of peaceful exploration in Article IV. Though these treaties reinforce the UN Charter’s prohibition on the use of force, the Outer Space Treaty remains critically flawed in its capacity to enforce this ban. [5] Article 103 of the UN Charter establishes that any provision in the Charter supersedes any international agreement, including the Outer Space Treaty. [6] Consequently, Article 51 continues to remain an effective loophole against both the prohibition on the use of force and the peaceful nature advocated by the Outer Space Treaty. As such, it is vital to consider how Article 51 is regulated under current international law and to examine the future implications of this critical gap.
Before delving into the specifics of how Article 51’s use applies to conflicts in space, it is crucial to establish the Charter’s jurisdiction. According to the Outer Space Treaty, even as individual states cannot lay sovereign claim to any celestial body in space, they retain nationality over their personnel and launch crafts. [7] Thus, any object launched into space is considered an extension of the state’s nationality. This claim is further bolstered by an analysis of the United Nations Convention on the Law of the Sea (UNCLOS). Similar to outer space, international waters contain sectors such as the high seas where no nation may lay sovereign claim. Nonetheless, navigating these waters requires identifying nations and resolving conflicts in these regions. As such, Article 91 affirms that states can establish their nationality by clearly flying the state’s flag on the vessel. [8] When applying both the explicit legislation of UNCLOS and the principles of the Treaty Governing the Activities of States, it becomes evident that states can retain sovereignty over their vessels in outer space. Flying a flag on a rocketship or satellite indicates a retention of nationality, which would remain subject to the articles of the UN Charter, including Article 51. This context is crucial because it demonstrates the potential for the use of force. If states can justify that an attack against their mission personnel or launch craft constitutes an armed attack, this may be a strong enough ground for invoking Article 51, potentially triggering a conflict that would have disastrous effects on the inherently peaceful nature of outer space.
Because the UN Charter has yet to define what legitimizes an armed attack, the scope of the right to self-defense can be interpreted through the principles of necessity and proportionality, which the ICJ previously established as customary law in Nicaragua v. the United States and Oil Platforms. These principles help distinguish the types of conflict that may lead to the successful invocation of Article 51. The Case Concerning the Military and Paramilitary Activities in and Against Nicaragua (Nicaragua v. United States of America) (1986) was the first major case to determine a framework regarding the use of Article 51. In 1984, Nicaragua filed a suit against the United States for the unlawful armed support of rebel groups in the country. In particular, the US was charged with supporting the use of force against the Nicaraguan government. In response, the US claimed the Nicaraguan government was illegitimately supplying arms to rebel forces in El Salvador, provoking an “armed attack” against El Salvador. [9] On this ground, the US argued that this attack provided justification for the use of force against the Nicaraguan government via support to rebel groups within Nicaragua as a form of collective self-defense by the US and El Salvador. Ultimately, the court ruled that the US had violated the prohibition on the use of force. The ruling signified that there needed to be certain limits based on international norms that provided a more concrete basis for testing whether an armed attack was justified or not. In particular, the court made references to two key principles of customary international law: necessity and proportionality. The court’s decision constrained future invocation of Article 51 by establishing these principles as a standard for determining armed attack. As the judgment states:
“The United States must also show that its actions were necessary and proportional to the armed attack made on it, and that the platforms were a legitimate military target open to attack in the exercise of self-defence.” [10]
The utilization of customary international law in determining the measure of an armed attack was upheld again in the Islamic Republic of Iran v. The United States of America (Oil Platforms) (1992). Following a series of attacks on Iranian oil platforms, Iran alleged that the US had breached numerous provisions of international law prohibiting the use of force. In a counter-suit, the US similarly claimed that these attacks were a response to previous attacks by Iran on vessels in the Gulf and interference by the Iranian government in commercial operations between Iran and the US. Nonetheless, the Court reaffirmed that the USs failed to demonstrate the necessity and proportionality of its attacks on the oil platforms to successfully constitute self-defense. As the Court stated:
“The United States could exercise such a right of self-defence only if it had been the victim of an armed attack by Iran and the United States' actions must have been necessary and proportional to the armed attack against it.” [11]
The Oil Platforms decision built upon the framework originally implemented by Nicaragua. It further limited the use of Article 51 by illustrating that not only did a use of force have to be essential to the state’s security, but the attack itself had to be proportional to the damage done by another state. Together, Nicaragua and Oil Platforms establish the conditions of proportionality and necessity when considering the justification of an invocation of Article 51. As both principles are designated international norms, the two are effectively restrictive principles within which Article 51’s use in outer space must be considered. [12]
Though the realm of outer space continues to be dangerously unregulated, international customary norms can serve as an effective measure for managing unjustified uses of force in space. The principles of proportionality and necessity are reliable tests that allow bodies such as the ICJ to provide compensation for nations where Article 51 has been unjustifiably invoked to allow the use of force. Nonetheless, these crucial gaps in international law must be bridged through explicit legal text. While necessity and proportionality are applicable for redressing harm, they are not capable of providing preventative measures. In an arena where peaceful measures are vital for scientific exploration, the use of force is an imminent threat. For example, in May of 2024, the Russian government launched a satellite called Cosmos 2576 into low Earth orbit. Since then, the US has claimed the satellite to be a counter-space weapon that could potentially attack other satellites. Furthermore, the US government perceives the satellite as an indication of Russian weaponry being developed to attack American satellites, highlighting the growing tensions that could lead to a future weaponization of space. [13] Necessity and proportionality are foundational norms that can successfully prevent these detrimental satellite conflicts in space by establishing a framework for operating the use of force. Even so, it is apparent that international law desperately requires a more comprehensive regulatory framework for outer space, one that simultaneously addresses the dangers of the increasing use of outer space while maintaining the key principles of peaceful scientific exploration defined in the Outer Space Treaty.
Edited by Megan Chopra and Claire Thornhill
[1] Wilmshurst , Elizabeth. “Ukraine: Debunking Russia’s Legal Justifications.” Chatham House, February 24, 2022. https://www.chathamhouse.org/2022/02/ukraine-debunking-russias-legal-justifications.
[2] United Nations. “United Nations Charter.” n.d. https://www.un.org/depts/los/convention_agreements/texts/unclos/unclos_e.pdf.
[3] Biersteker, Thomas. “Global Challenges - The Weaponisation of Economics.” Global Challenges, November 18, 2022. https://globalchallenges.ch/issue/12/sanctions-against-russia-and-the-role-of-the-united-nations/.
[4]Biersteker, “Global Challenges”.; United Nations. “Treaty Governing Activities of States in the Exploration and Use of Outer Space Including the Moon and Other Celestial Bodies.” n.d. https://www.unoosa.org/pdf/gares/ARES_21_2222E.pdf
[5] United Nations, “Treaty Governing Activities of States”.
[6] United Nations, “United Nations Charter”.
[7] United Nations, “Treaty Governing Activities of States”.
[8] United Nations. “United Nations Convention on the Law of Sea”. n.d. https://www.un.org/depts/los/convention_agreements/texts/unclos/unclos_e.pdf. Accessed 21 Oct. 2025
[9] “Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States of America).” June 27, 1986. https://www.icj-cij.org/case/70.
[10] “Military and Paramilitary Activities”.
[11] “Oil Platforms (Islamic Republic of Iran v. United States of America).” November 6, 2003. https://www.icj-cij.org/case/90.
[12] “Rule 14. Proportionality in Attack.” March 2025. https://ihl-databases.icrc.org/en/customary-ihl/v1/rule14.
[13] “US Says Russia Likely Launched Space Weapon.” May 22, 2024. https://www.bbc.com/news/articles/cq55ww5j7e2o.