Assessing the Legality of Israel’s Action Against Iran Under International Law

As related in previous Articles of War coverage (here, here, and here), in the early hours of 13 June, Israel launched “Operation Rising Lion” which struck Iran’s nuclear enrichment facilities in Natanz, ballistic facilities, military commanders, and nuclear scientists. According to a statement by Israel’s Prime Minister, it was “a targeted military operation to roll back the Iranian threat to Israel’s very survival.” He added that the operation will continue for “as many days as it takes to remove this threat.”

Iran and its determination to develop nuclear weapons are viewed by Israel as existential threats. Former Iranian President Mahmoud Ahmadinejad for example called Israel a “disgraceful blot” that should be “wiped off the face of the earth.” Its Supreme Leader, Ayatollah Ali Khamenei, called Israel a “cancerous tumor” that “must be eradicated.”

A few days earlier, the International Atomic Energy Agency (IAEA) Board of Directors declared that Iran was in breach of its non-proliferation obligations following a report circulated on May 31 which found undeclared locations, material, and activities related to Iran’s nuclear programme which can be used to also make nuclear weapons (see also here and here ). As in previous cases, Iran’s response to such reports was further escalation.

According to an Israel Defence Forces (IDF) statement, intelligence showed that Iran has significantly advanced towards achieving nuclear capability and ability to attack Israel. Senior nuclear scientists have secretly conducted experiments required to build nuclear weapons. It also noted that the goal of Israel’s action was to eliminate an Iranian “plan to destroy Israel that has taken shape in recent years” and the operation took place “at the most appropriate time in light of an existing and real threat and with the highest military readiness.”

Israel affirmed these details in its letter to the Security Council. Israel’s Foreign Minister also said that the action was necessary to “remove from Israel the threat of annihilation.”

Israel’s recent actions raise significant geopolitical questions. However, this post focuses on evaluating the legal framework within which these actions occurred. The central issue under examination is whether Israel’s actions can be justified under the rubric of self-defence. I will argue that they fall within the scope of lawful self-defence as recognized under Article 51 of the UN Charter and customary international law. It should be noted, however that this is one approach to assessing the legality of the operation; an alternative approach is that Israel’s actions are part of an ongoing armed conflict and thus does not require an ab nuovo jus ad bellum justification. This view is also supported not only by Israel’s letter to the Security Council  but also by commentators.

This post will examine the scope of the concept of imminence in the context of self-defence and apply that analysis to the present case, arguing that Israel’s actions constituted a lawful response to an imminent attack by Iran. In this context, it will also consider the lawfulness of targeting nuclear scientists during Israel’s self-defence action.

The Right to Self-Defence and Imminence

Article 51 of the UN Charter affirms the right to self-defence “if an armed attack occurs.” A strict interpretation confines this right to scenarios where an armed attack has already materialized. This view has been supported by the International Court of Justice, which adopted a narrow reading in the Paramilitary Activities case, emphasizing that the use of force must be predicated on an actual armed attack (para. 195).

However, Article 51 and customary law permit the use of force in self-defence even before an armed attack has occurred based on criteria set out in the Caroline incident, namely a “necessity of self-defence, instant, overwhelming, leaving no choice of means and no moment of deliberation.” There is now widespread consensus among scholars and States supporting this form of self-defence, often referred to as “pre-emptive” or “anticipatory” self-defence. In this context “imminence” is generally understood in temporal terms, referring to an immediate or reasonably foreseeable attack.

The requirement of temporal proximity of the armed attack has been increasingly challenged in contexts involving terrorism or weapons of mass destruction, where the consequences of waiting for an actual attack to materialize could be catastrophic. Contemporary interpretations of imminence converge on a more contextual approach, one that considers a range of factors beyond mere temporal proximity (see here, p. 63-64 and here, p. 97-307). This understanding of imminence can justify the use of defensive force when there is a “last window of opportunity” (see here and here p. 351-352).

For example, according to the Chatham House Principles of International Law on the Use of Force by States in Self-Defence,

Whether the attack is “imminent” depends upon the nature of the threat and the possibility of dealing effectively with it at any given stage. Factors that may be taken into account include: the gravity of the threatened attack—whether what is threatened is a catastrophic use of Weapons of Mass Destruction (WMD); capability—for example, whether the relevant State or terrorist organisation is in possession of WMD, or merely of material or component parts to be used in its manufacture; and the nature of the attack—including the possible risks of making a wrong assessment of the danger. Other factors may also be relevant, such as the geographical situation of the victim State, and the past record of attacks by the State concerned (p. 967-68).

Also, according to Principle 8 of the Bethlehem Principles,

Whether an armed attack may be regarded as “imminent” will … be assessed by reference to all relevant circumstances, including (a) the nature and immediacy of the threat, (b) the probability of an attack, (c) whether the anticipated attack is part of a concerted pattern of continuing armed activity, (d) the likely scale of the attack and the injury, loss, or damage likely to result therefrom in the absence of mitigating action, and (e) the likelihood that there will be other opportunities to undertake effective action in self-defense that may be expected to cause less serious collateral injury, loss, or damage. The absence of specific evidence of where an attack will take place or of the precise nature of an attack does not preclude a conclusion that an armed attack is imminent for purposes of the exercise of a right of self-defense, provided that there is a reasonable and objective basis for concluding that an armed attack is imminent.

This interpretation of imminence has been adopted by the United Kingdom, Australia, and the United States. Even if the positions of these States may not fully represent the views of the international community as a whole, they should not be dismissed merely because other States have not articulated their own interpretations of imminence. In my view, these positions provide a valuable foundation for understanding how the concept of imminence can be interpreted in light of contemporary security threats, particularly those involving WMD.

The legality of Israel’s action, therefore, hinges on whether an Iranian attack was imminent in its temporal or contextual construction. In my opinion it fulfils the imminence criterion. As was said, Israel invoked intelligence information supporting the view of Iran planning to attack Israel, with the Defence Minister characterising Israel’s response as an act of pre-emptive self-defence to avert the attack.

Notably, when asked in April 2025 about Iran’s nuclear capabilities, the IAEA Director General stated,

Iran is not far from having a nuclear weapon. They don’t have it. We know it. And I always try to repeat so as not to confuse people internationally. But the great difference is that the material for it is already there, to make a few warheads … . So of course, you know, dates are always arbitrary. But they are not far. It would be, you know, a matter of months, not years, indeed.

In its letter to the Security Council, Israel stated that the operation was a measure of last resort in order to thwart the threat of Iranian imminent attacks and that it was the last window of opportunity for Israel to protect its existence.

The prospective attack by Iran must also be viewed within the broader pattern of attacks directed at Israel by Iran and its proxies, including Hamas, Hezbollah, and the Houthis. These repeated attacks underscore Iran’s sustained intent to attack and harm Israel, a threat that would be significantly amplified if Iran were to acquire nuclear weapons.

The openly bellicose rhetoric of Iranian leaders further supports this conclusion. In my opinion, public declarations by Iranian officials should not be dismissed as mere domestic political posturing because, as recent indictments of Israeli officials show, they can have legal consequences.

Moreover, the nature of a potential nuclear strike and the catastrophic harm it could inflict are critical considerations when assessing imminence and consequently the legality of self-defence.

The International Court of Justice in its advisory opinion on the Legality of the Threat or Use of Nuclear Weapons indeed acknowledged, also in the case of nuclear weapons, it “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 of the Charter” (para. 96). The only viable opportunity to defend when threatened with a nuclear armed attack is to act before the attack occurs because acting after a nuclear strike may be practically impossible, rendering the right to self-defence effectively meaningless. It would be disingenuous—and legally and morally untenable—for international law to require a State to endure the devastation of a nuclear strike before invoking its inherent right to self-defence. To slightly paraphrase Eugene Rostow, international law is not a suicide pact.

It is true however that there is no scientific method of proving imminence; its determination depends on intelligence information and reasonable judgment. It cannot be denied however that the foremost responsibility of State leaders is to protect their populations who will also hold them into account. As Israel’s Foreign Minister noted, “In such situations, decision-makers must ask themselves not only what the gains and costs of military action will be, but also what the cost of inaction would be. In this case: placing Israel in danger of annihilation.”

While it is also desirable that claims of imminence be supported by clear and transparent evidence, the sensitive nature of intelligence often limits the extent to which such information can be publicly disclosed. Nonetheless, the absence of complete transparency should not lead to the automatic dismissal of these claims as implausible or unfounded.

Targeting Nuclear Scientists in the Context of Self-Defence

A pertinent question that also arises from the recent operations is whether the targeting of nuclear scientists can be legally justified. This issue falls within the framework of jus in bello, both for those who argue that Israel and Iran were already engaged in an ongoing international armed conflict and for those, like myself, who maintain that an armed conflict or at least a new armed conflict commenced on 13 June. With regard to the latter approach, while Israel’s initial strike on Iran would be governed by jus ad bellum, the execution of the self-defence action would be regulated by jus in bello (see also here and here).

The targeting of nuclear scientists, however, raises complex questions regarding their status. There has been speculation that at least certain scientists were members of Iran’s armed forces. If that is correct, their targeting is lawful under the law of armed conflict.

For those nuclear scientists who retained their civilian status, however, the key question becomes whether they directly participated in hostilities. This requires a case-by-case analysis. The International Committee of the Red Cross’s Interpretive Guidance on the Notion of Direct Participation in Hostilities offers an initial framework for this assessment. In that work, the participation of scientists, and for this reason nuclear scientists, is characterised as indirect rather than direct.

However, it is important to note that the Guidance is non-binding, and not all of its conclusions have been widely accepted or elevated to the level of customary international law. During the debates preceding its adoption, it was argued that, in exceptional circumstances, scientists’ involvement may qualify as direct participation in hostilities (see here, n. 122). The U.S. Department of Defense Law of War Manual also takes a more contextual approach to direct participation in hostilities (DPH) by taking into account, among other things: the purpose underlying the act as well as the military significance of the activity to the party’s war effort, in particular the degree to which the act contributes to a party’s military action against the opposing party; whether the act is of comparable or greater value to a party’s war effort than acts that are commonly regarded as taking a direct part in hostilities; whether the act poses a significant threat to the opposing part (§ 5.8.3; see also here, p. 238). Professor Michael N. Schmitt, a leading expert on the subject of DPH, also argued in a different context that the criteria should consider whether an individual performs an indispensable function in making possible the application of force against the enemy.

In my view, this approach is applicable to nuclear scientists actively contributing to the development of nuclear weapons. Their contribution is non-fungible—meaning their expertise cannot be easily or automatically replaced—thus establishing a critical and direct link to Iran’s operational and subsequently tactical nuclear capability. Indeed, two of the targeted nuclear scientists were described by Iran as “major nuclear scientists.” Also, according to the IDF, Iran “has established a secret program, within the framework of which senior nuclear scientists in Iran secretly conducted the experiments required to build nuclear weapons.” This means they contribute to a programme designed to cause the harm and there is belligerent nexus because their contribution takes place with a specific conflict in mind. This unique role justifies their targeting under the DPH criteria (although for a contrary conclusion, see here).

Conclusion

Although the conflict between Israel and Iran undoubtedly raises significant geopolitical questions, this post has focused on the legal dimension, arguing that Israel’s use of force constitutes a lawful act of self-defence. It meets the criterion of imminence not only under the temporal interpretation but even more convincingly under its contemporary, contextual understanding.

The post also contended that while self-defence provides a jus ad bellum justification for the initial use of force, the legality of targeting nuclear scientists must be assessed under the law of targeting. Such targeting may be lawful if the individuals in question are integrated into Iran’s armed forces or if they directly participate in hostilities.

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Nicholas Tsagourias is Professor of International Law at the University of Sheffield, UK.

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.

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