Deterrence Doesn’t Fly in Space: Nuclear Weapons in Outer Space as a Threat of Force
In an interview with German newspaper Die Welt on Friday April 12th, NATO Secretary-General Rutte expressed concern regarding the potential deployment of Russian nuclear weapons in orbit around Earth. While this possibility had already surfaced in 2024, as reported by the Washington Post, this marks the first time a world leader has publicly addressed its concerns regarding the issue. It should come as no surprise that the NATO Secretary-General has chosen to speak out. Whereas Putin has denied plans to deploy a nuclear weapon in space, consistent warnings from intelligence agencies suggest a far more troubling reality.
A nuclear weapon in outer space could, with a single detonation, disable thousands of satellites. The resulting disruption would cripple global communication, navigation, and financial systems, undermining not only international security, but also the daily functioning of societies worldwide. To put it bluntly, if a significant portion of satellites was destroyed, you might not even be able to read this post.
This post will examine whether the stationing of a nuclear weapon in outer space could constitute a “threat of force” under international law and is therefore inherently illegal.
Space Law
Space law serves as the lex specialis governing activities in outer space, with the Outer Space Treaty (OST) forming its cornerstone. The OST, which entered into force in 1967 amid Cold War tensions, currently has 102 parties, including major powers such as Russia, China, and the United States. Article IV of the treaty includes a clear and uncontested prohibition: nuclear weapons may not be placed in orbit around the Earth, stationed on celestial bodies, or otherwise stationed in outer space. This provision reflects a rare point of consensus among States, including those that are nuclear-armed, and demonstrates an early recognition of the destabilizing potential of space-based nuclear armaments.
As a primary rule of international law, a breach of Article IV could allow injured States to take lawful countermeasures in response. Major Brian Green explores this dimension in detail in his recent Articles of Warpost. Notably, the OST enjoys broader membership than other nuclear-related treaties, such as the Treaty on the Non-Proliferation of Nuclear Weapons or various non-proliferation agreements. This widespread ratification suggests that, at least on paper, the non-proliferation of nuclear weapons in outer space has been a shared and undisputed objective among the international community.
Threat of Force
While international law recognizes no strict hierarchy of norms, analyzing this issue under the framework of Article 2(4) of the UN Charter—rather than just the Outer Space Treaty—offers a broader and more fundamental legal lens. The Outer Space Treaty also expressly incorporates the UN Charter in Article II. The stationing or testing of nuclear weapons in orbit around Earth may constitute a threat of force, engaging the foundational principles of the international legal order. These norms are not only treaty-based but widely regarded as customary international law, binding on all States.
The “threat” framework thus draws attention not only to a treaty violation, but to the broader destabilizing effects such conduct may have on international peace and security. It also expands the legal conversation. Unlike the OST, which binds only its parties, the prohibition on the threat or use of force applies universally. Shifting the focus from a narrow treaty violation to a potential breach of a core principle of the UN Charter enhances both the legal significance and the strategic urgency of the issue.
In addition to the more well-known prohibition on the use of force, Article 2(4) of the UN Charter contains a prohibition on the threat to use force. Specifically, it provides,
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 manner inconsistent with the Purposes of the United Nations.
Although State practice explicitly invoking the prohibition on threats of force is limited, the most deliberate discussion of this norm appears in the International Court of Justice (ICJ) 1996 Advisory Opinion on the Legality of the Threat or Use of Nuclear Weapons. In this opinion, the Court was asked to answer the question: “is the threat or use of nuclear weapons in any circumstance permitted under international law?” In addressing this question under Article 2(4) of the Charter, the Court examined nuclear deterrence and the signaling of nuclear capabilities.
In paragraph 47 of the opinion, the Court stated,
Whether a signaled intention to use force if certain events occur is or is not a ‘threat’ within Article 2, paragraph 4, of the Charter depends upon various factors. If the envisaged use of force is itself unlawful, the stated readiness to use it would be a threat prohibited under Article 2, paragraph 4.
The ICJ elaborated on this further in paragraph 48.
Some States put forward the argument that possession of nuclear weapons is itself an unlawful threat to use force. … Whether this is a ‘threat’ contrary to Article 2, paragraph 4, depends upon whether the particular use of force envisaged would be directed against the territorial integrity or political independence of a State, or against the Purposes of the United Nations or whether, in the event that it were intended as a means of defense, it would necessarily violate the principles of necessity and proportionality.
Accordingly, the legality of a threat hinges on the legality of the use of force it implies. Neither the ICJ nor Article 2(4) requires a threat to be made explicitly, such as through verbal or written statements. Rather, a threat may be implicit, and the key legal test is whether the force signaled would be lawful under international law.
Are Nuclear Weapons in Outer Space Permissible under International Law?
The central question is whether the orbital deployment and use of nuclear weapons in outer space can ever be considered permissible under international law. In the Nuclear Weapons Advisory Opinion, the ICJ reviewed various treaties and concluded that, as a general matter, no comprehensive prohibition on nuclear weapons exists that is universally ratified, particularly not by the nuclear-armed States. As the Court noted in paragraph 52, “State practice shows that the illegality of the use of certain weapons as such does not result from an absence of authorization but, on the contrary, is formulated in terms of prohibition.”
In other words, in the absence of an express legal prohibition, the use of certain weapons is not per seunlawful. However, this legal position is significantly different in outer space. As previously discussed, Article IV of the Outer Space Treaty includes a clear and explicit prohibition on the placement of nuclear weapons in orbit or otherwise stationed in outer space. It is an obligation accepted by all major space powers, including Russia and the United States. This prohibition is not vague. It is a binding, unambiguous ban on placing nuclear weapons in orbit.
From this, a crucial inference follows. Because any orbital stationing of nuclear weapons is categorically prohibited, any threat to place such weapons in that domain would necessarily involve the envisaged use of unlawful force. Following the reasoning of the ICJ, this makes the threat itself unlawful, considering it as a grave violation of the purposes of the UN Charter regarding international peace and stability. In short, the legal regime governing outer space renders the placement of a nuclear weapon not only a treaty violation, but also an inherently illegal threat to use force under Article 2(4) of the UN Charter.
Deployment versus Threat
Indeed, the act of deployment itself can not necessarily be considered as an “automatic” threat. This is also seen in the Court’s reasoning in the Advisory Opinion in paragraph 47 above. “Whether a signaled intent to use force is or is not a threat under Article 2, paragraph 4, of the Charter depends upon various factors…” (emphasis added). As such, the Court relies on an analysis of intent to use nuclear weapons as crucial in the assessment of its illegality as a threat. Nuclear deterrence on Earth is clearly allowed. There is no widespread opinio juris that signals a prohibition on the deployment, development, or stationing of nuclear weapons on Earth where the declared nuclear posture of States envisions the use of nuclear weapons in conformity with the Charter, i.e.., in self-defense, proportionality and only when necessary. As such, it is a diplomatic tool to keep States in check. However, such a consideration in outer space is markedly different.
While a threat is not presumed automatically to demonstrate an intent to use such force in every case of military deployment, the specific nature of space-based nuclear weapons—combined with their clear prohibition under the Outer Space Treaty—makes it difficult to interpret such deployment as anything but a credible, unlawful and intentional threat to use force. Considering the significant costs of development, deployment and maintenance of a nuclear weapon in outer space, the act of deployment of such a weapon in outer space strongly implies a readiness to use that weapon. In this sense, the legal and strategic environment of outer space narrows the interpretative gap between “deployment” and “threat,” tipping the legal analysis toward a violation of Article 2(4) UN Charter, in addition to a violation of Article IV Outer Space Treaty, by default.
Self-Defense?
Of course, the right of self-defense remains a cornerstone of international law. As explicitly recognized by the Article 51 of the UN Charter, “Nothing in this present Charter shall impair the inherent right of individual or collective self-defense if an armed attack occurs against a Member of the United Nations … .” In the Nuclear Weapons Advisory Opinion, the Court reaffirmed this principle regarding nuclear weapons in paragraph 96.
Furthermore, the Court cannot lose sight of the fundamental right of every State to survival, and thus its right to resort to self-defence, in accordance with Article 51, when its survival is at stake.
The ICJ ultimately concluded in paragraph 97,
[T]he Court … cannot reach a definitive conclusion as to the legality or illegality of the use of nuclear weapons by a State in an extreme circumstance of self-defence in which its very survival would be at stake.
In other words, the Court left open the possibility that the threat or use of nuclear weapons might be lawful in extreme scenarios where a State’s very survival is imperiled in the exercise of lawful self-defense.
However, this reasoning becomes far more difficult to sustain in the context of outer space. This is because a lawful exercise of self-defense remains constrained by proportionality and necessity. Unlike terrestrial (nuclear) uses of force which might satisfy these conditions, the effects of nuclear detonations in space likely have indiscriminate and disproportionate effects. A single nuclear blast could disable or destroy hundreds, if not thousands, of satellites, including those belonging to third States. Its effects would not be confined to the attacker alone but would radiate across the entire international community. Given this impact, it is difficult to envision a scenario in which the use of nuclear weapons in space could ever satisfy the legal requirements of necessity and proportionality for lawful self-defense.
Even if a State claimed that the use of a nuclear weapon in outer space was essential to its survival, the scale and uncontrollability of the resulting damage would likely render the response unlawful. Consequently, the threat to use such a weapon—including through its placement in orbit—would still amount to an unlawful threat to use force under Article 2(4) of the UN Charter. As a result, no scenario can be imagined where the orbital deployment of a nuclear weapon in outer space would be compliant with the norms of international law.
Conclusion
The prospect of deploying nuclear weapons in outer space is not only alarming from a strategic standpoint. It is legally indefensible. While terrestrial nuclear deterrence occupies a tolerated space in international law and relations, the legal framework governing outer space is far more definitive.
The Outer Space Treaty’s explicit prohibition on the placement of nuclear weapons combined with the UN Charter’s foundational ban on the threat or use of force creates a concluding legal standpoint: nuclear deployment in outer space cannot be dismissed as a neutral act of deterrence. Instead, it represents both a clear treaty violation and an illegal threat to use force. Even where self-defense is involved, the indiscriminate and uncontrollable effects of nuclear detonations in space make compliance with necessity and proportionality virtually impossible.
In short, the law is clear: nuclear weapons do not belong in space. Any move to place them there must be understood not only as a diplomatic escalation, but as a legal violation of the highest order.
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Louisa Handel-Mazzetti is an Assistant Professor at the Royal Netherlands Defense Academy (NLDA) and a PhD Candidate at the Institute of Air and Space Law at Leiden University.
The views expressed are those of the author, and do not necessarily reflect the official position of the United States Military Academy, Department of the Army, or Department of Defense.
Articles of War is a forum for professionals to share opinions and cultivate ideas. Articles of War does not screen articles to fit a particular editorial agenda, nor endorse or advocate material that is published. Authorship does not indicate affiliation with Articles of War, the Lieber Institute, or the United States Military Academy West Point.
Photo credit: Unsplash, SpaceX
