Missiles and Messages: Characterizing the Use of Force between Iran and Israel

[Masoud Zamani is a lecturer in International Law and International Relations at the University of British Columbia, Canada. 

Amir Abbas Kiani is a collaborating researcher in International Law at Shiraz University, Iran.]

The recurring exchange of lethal force and military operations between Iran and Israel gives rise to several urgent and complex legal questions within the framework of jus ad bellum. A particularly intriguing feature of these confrontations is their episodic nature, prompting debate over whether they are better understood as anticipatory self-defense or unlawful reprisals. On the one hand, official rhetoric from both governments often frames these actions as retaliatory, supporting the interpretation that they are reprisals. On the other hand, the deliberate targeting of military objectives suggests a strategic rationale consistent with anticipatory self-defense. Distinguishing between the two is critical: While reprisals involving force are generally prohibited under international law, anticipatory self-defense occupies a more ambiguous and contested legal space.

Although reprisals are widely considered unlawful under jus ad bellum, this view is not without its challengers. Some argue that the frequency of such actions in state practice raises doubts about whether a categorical prohibition has truly crystallized under customary international law. In the context of the Iran-Israel conflict, one commentator has noted that the repeated resort to retaliatory strikes may lend weight to the claim that reprisals are not unequivocally banned: “[W]hat a state does in retaliation for such one-off violation of its sovereignty is assessed and judged in relation to precedents reflecting past practice. If deemed to be consistent with such practice, it is legitimized”.

Anticipatory self-defense, by contrast, refers to a state’s right to use force in anticipation of an imminent armed attack. Over the past two decades, this doctrine has frequently been invoked in discussions about the evolving security dynamics between Iran and Israel. In the current context, characterizing the recent use of force requires a careful legal analysis of each side’s claims and actions.

In the following discussion, we will examine the arguments supporting both interpretations—reprisals and anticipatory self-defense—and explore the potential legal characterizations available for recent Iran-Israel clashes under international law.

Rhetoric of Punishment

The current escalation in the Iran-Israel conflict can arguably be traced back to April 1, 2024, when an airstrike near the Iranian embassy in Damascus killed a high-ranking officer of the Islamic Revolutionary Guard Corps (IRGC). In retaliation, Iran launched an unprecedented direct attack on Israeli territory on April 13, deploying a barrage of missiles and drones. This was followed by an Israeli strike on an Iranian missile platform in Isfahan on April 19. The targeted killing of Hamas leader Ismail Haniyeh in Tehran, and later the assassination of the Secretary General of Hezbollah Seyyed Hassan Nasrallah in Beirut, provoked Iran to launch another missile attack in October. Israel then responded with a new wave of strikes against military installations within Iran. The conflict has since entered a temporary lull, with the constant risk of sudden re-escalation.

Both Iran and Israel have been exchanging vows of retaliation since at least April 1, 2024. Following the airstrike in Damascus, the Supreme Leader of the Islamic Republic, Ayatollah Ali Khamenei, vowed revenge: “The Zionist regime will be punished by the hands of our brave men. We will make it regret this crime and others it has committed.” In the wake of Iran’s missile and drone barrage on April 13, 2024, Gen. Herzi Halevi, Chief of Staff of the Israel Defense Forces (IDF), declared that “the launch of so many missiles and drones to Israeli territory will be answered with a retaliation.” After the killing of Hamas’s leader, Ismail Haniyeh, in Tehran on July 31, 2024, “Iran threatened ‘harsh punishment’ for Israel.” Again, following Iran’s attack on Israel on October 1, 2024, Israeli Prime Minister Benjamin Netanyahu “called Iran’s massive missile attack on Israel ‘a big mistake’ and vowed to make Tehran ‘pay for it’.” 

The language employed by officials from both Iran and Israel clearly reflects an intent rooted in punishment and retaliation. Such statements complicate the legal justification of these actions, as they are openly framed in terms of retribution. This rhetoric is not without consequence. International law prohibits not only acts of aggression but also threats thereof. Still, the legal implications of rhetorical threats cannot be equated with the actual conduct on the ground. In many cases, such threats serve domestic political purposes, aiming to rally public support for future uses of force (see e.g., here). Therefore, to arrive at an accurate legal characterization of these measures, our analysis must ultimately be grounded in the facts and circumstances of the operations themselves.

Reprisals and Defensive Use of Force  

Armed reprisals are traditionally defined as acts of counter-force, undertaken by a state in response to a prior violation of international law (see e.g. pp. 244–245). The United Nations (UN) Charter does not explicitly prohibit reprisals. However, a cumulative reading of Articles 2(4) and 51 leads to the conclusion that the Charter intends to restrict the use of unilateral force solely to instances of self-defense. This interpretation has been reaffirmed by various international bodies. The UN General Assembly (see e.g. p. 122), the UN Security Council on multiple occasions (see e.g. p. 10), and the International Court of Justice (ICJ) (para. 46) have all declared that armed reprisals are prohibited under the jus ad bellum framework.

The core distinctions between self-defense and armed reprisals relate to the timing and purpose of the use of force. While the most restrictive view of self-defense confines it to repelling an ongoing armed attack (p. 400), an armed reprisal occurs after the initial attack has ceased and is primarily intended to punish the aggressor. However, this rigid distinction fails to account for situations in which reprisals may serve a defensive function. It is therefore reasonable to argue that if the functional equivalent of a reprisal falls within the broader interpretive boundaries of self-defense, it may not necessarily be unlawful.

This ambiguity stems both from the flexible interpretability of the self-defense doctrine and the customary uncertainty surrounding the precise boundary between lawful self-defense and unlawful reprisals. As has been acknowledged, “on several occasions, force was used in response to past events, rather than current or imminent armed actions, sometimes with considerable delay” (p. vii).

The timing and purpose of a forcible response are closely interconnected, aiming to ensure “that the right to use force in self-defence is protective in nature”, rather than punitive (p. 255). Still, it has been argued that “[s]ome level of preparation may be needed for the attacked State to mount an appropriate and coordinated response or to collect intelligence on the source and specifics of the attack… or it may engage in a time-consuming political decision-making process at the domestic level before actually giving a green light to the armed forces” (p. 100). With respect to timing, Michael Schmitt correctly observes that the key question is “whether there still is a need to respond forcibly at the time of the response.” This is often treated as a separate self-defense requirement—immediacy—and compliance with this condition helps distinguish a lawful act of self-defense from an unlawful one.

In short, “a defensive use of force may not be too late (immediacy)”. To clarify, immediacy refers to the temporal connection between the armed attack and the response—it concerns how soon a state must act in response to an actual attack. In contrast, imminence refers to the certainty of a threatened attack. It requires that the threat be impending and unavoidable unless action is taken. In its Nicaragua judgment, the ICJ rejected the United States’ claim of collective self-defense, noting that 

measures were only taken, and began to produce their effects, several months after the major offensive of the armed opposition against the Government of El Salvador had been completely repulsed (January 1981), and the actions of the opposition considerably reduced in consequence…(para. 237).

Anticipatory Self-Defense

While Article 51 of the UN Charter appears to require the actual occurrence of an armed attack to trigger the right of self-defense, it has been suggested that “[s]tates need not necessarily await the occurrence of an armed attack” (p. 255). Indeed, in interpreting the language of Article 51 of the UN Charter, some have maintained that the “‘if’ condition is met once an armed attack is under way, even if the attack is not yet fully matured” (p. 206). This practice, characterized as ‘anticipatory self-defense’, highlights that use of force “is permitted only when a further attack is both temporally close and with a high level of certainty” (pp. 57–58).

The key issue is how imminence and immediacy, as two components of the necessity requirement, should be distinguished—and which of the two should carry greater weight. This question becomes particularly complex in protracted conflicts characterized by recurring hostilities. In such contexts, immediacy, as a backward-looking concept tied to a prior armed attack, may risk transforming a lawful act of self-defense into an unlawful armed reprisal. Conversely, imminence, as a forward-looking notion, may be used to justify anticipatory self-defense in the face of a perceived impending threat. The cyclical nature of violence in some conflicts—marked by intermittent pauses between escalations—can blur the lines between reprisal, anticipatory self-defense, and lawful use of force, making it difficult to articulate a clear legal framework. This is particularly the case in the context of the Iran-Israel conflict, where these temporal elements of necessity frequently overlap and intensify interpretive challenges.  

It has been argued that for

states that are responding to prior armed attacks with the stated objective of preventing the occurrence of additional attacks… the ability to react defensively reflects the prospective element of necessity, which allows defensive action to prevent further anticipated attacks from the same source (pp. 304-305) (emphasis added).

As regards the issue of ‘prior armed attacks’, two possible scenarios can be imagined. In the first, the forcible response is viewed as a reaction to an ongoing series of attacks, falling under the ‘ongoing attack’ paradigm. As the International Law Association (ILA) has elaborated, “the risk of further attacks can be seen as a continuation of the initial armed attack and prevention of these being a part of the same self-defence action” (p. 11). In the second scenario, the prior armed attack is treated as an indication that further attacks may be imminent. The first scenario aligns with the traditional understanding of lawful self-defence, as it responds to an ongoing armed attack. The second, by contrast, falls within the realm of anticipatory self-defence, whose legitimacy under international law hinges on strict criteria. To be lawful, such action must be taken in response to a threat that is not only serious but also truly imminent. However, if the primary aim is preventive—seeking “to deprive an adversary of its future ability to mount an armed attack”—then the use of force is generally deemed unlawful (here and here). This implies that for anticipatory self-defense to be lawful, it must be exercised at the “last possible window of opportunity” in response to an attack that is almost certain to occur.

Iran-Israel Confrontations: Facts on the Ground

If either Iran or Israel sought not to punish but to prevent future imminent attacks by the other side, one might argue that what appears to be an armed reprisal could in fact constitute anticipatory self-defense. For such a claim to hold, however, the acting state must demonstrate a reasonable belief that another attack was imminent. Notably, both Iran and Israel appear to have limited their operations to military targets (see e.g., here and here). This pattern may be interpreted as evidence of an anticipatory—not retaliatory—purpose. That is, both states could plausibly argue that their intent was to degrade the other’s military capabilities in order to forestall further attacks. 

It must be noted that punitive intent remains key in distinguishing reprisals from anticipatory self-defense. As has been noted, “punishment should not be the ultimate purpose for continued use of armed force. This would turn the military action into more of a reprisal, rather than an act of self-defense” (p. 88). It is clear that a certain degree of intention to punish the adversary can be discerned in the military engagements of both states in the Iran-Israel conflict. Needless to say, the more explicit the punitive undertone of an attack, the greater the likelihood that it will be classified as a retaliatory act—and thus subject to prohibition under international law. In the case of Iran and Israel, the timing of their respective strikes and the language of punishment lend support to the hypothesis that some of these operations may constitute reprisals. If these attacks are framed as reprisals, contrary to the suggestions mentioned above by some scholars, they should not be interpreted as evidence that reprisals are being legalized through recurring practice between two states, in violation of a norm with jus cogens status.  

Conclusion: Between Doctrine and Reality 

Ultimately, the Iran–Israel confrontation reveals the interpretive elasticity of core concepts within jus ad bellum. While official statements on both sides are steeped in the language of retribution, the operational details—particularly the choice of targets and the tempo of response—complicate any straightforward classification as unlawful reprisals. The doctrine of anticipatory self-defense, though contested, offers a legal lens through which some of these actions may be viewed, provided the conditions of imminence and necessity are met. In practice, the boundary between legitimate defense and prohibited reprisal remains porous, shaped less by doctrine than by evolving patterns of state behavior. As this dynamic continues to unfold, legal assessments must remain anchored not in rhetoric, but in facts on the ground and the enduring principles of international law.