Rethinking Immediacy in Israel’s Right of Self-Defense

On June 13, Israel initiated “Operation Rising Lion,” striking Iran’s nuclear facilities. According to Israeli President Benjamin Netanyahu, the attack was intended to stomp out Iran’s existential threat to Israel’s security: “We can’t leave these threats for the next generation. If we don’t act now, there won’t be a next generation.” Further, in his June 17 letter to the United Nations, the Israeli foreign minister described Iran’s nuclear and ballistic missile programs as “existential and imminent” threats. Since then, there has been considerable debate about whether Operation Rising Lion constituted a lawful act of anticipatory self-defense.

However, a separate and independent legal basis for Israel’s use of force exists in Iran’s missile attacks against Israel in April and October 2024. During those months, Iran launched large-scale missile and drone attacks directly against Israel, firing hundreds of ballistic, hypersonic, and cruise missiles at military installations and urban areas. The attacks caused civilian casualties, property damage, and widespread disruption, despite Israel’s successful interception of most incoming projectiles. Those strikes qualified as an “armed attack” within the meaning of Article 51 of the United Nations Charter and triggered an international armed conflict (IAC) between the two states. Israel was entitled to rely on the 2024 attacks as the legal foundation for its June 2025 resort to force.

The passage of six months between Iran’s 2024 missile attacks and Israel’s resort to force on June 13 in no way extinguished Israel’s right to invoke those earlier attacks as the legal basis for the use of force in self-defense. That right remained operative, notwithstanding that direct hostilities between Iran and Israel had abated until Israel launched its attack in June. Once the right to self-defense attaches—that is, once a state has been subjected to an armed attack that triggers the lawful exercise of self-defense under Article 51 of the UN Charter—it does not expire merely because a significant amount of time passes before the state chooses to respond. The question is not whether Israel acted promptly following the 2024 attacks, but whether, as of June, Operation Rising Lion constituted a necessary response to a continuing threat.

Israel and Iran’s International Armed Conflict Existed Before Operation Rising Lion

Iran’s 2024 missile attacks against Israel marked a highly escalatory shift from proxy warfare to direct state-on-state armed engagement. Israel responded shortly thereafter by striking Iranian air-defense and radar installations, missile production sites, UAV factories, and a nuclear research facility. The scale and intensity of the 2024 attacks surpassed the relatively low threshold required to give rise to an international armed conflict. 

Under the doctrine of continuous self-defense, once a state lawfully invokes the right of self-defense in response to an armed attack, it may continue to use force against the aggressor without needing to independently justify each subsequent use, so long as the original threat persists. In such cases, further uses of force are governed not by jus ad bellum, which concerns the legality of a state’s initial resort to force, but by jus in bello, the body of international humanitarian law that governs the methods by which warfare may lawfully be conducted once an armed conflict is already underway. As Israel was already engaged in a lawful exercise of self-defense, its June 2025 offensive did not require a renewed jus ad bellum justification, but instead had to comply with the applicable rules governing the means and methods of conducting armed conflict.

Some analysts have suggested that the international armed conflict ended after October 2024 due to a lull in direct hostilities. That view overlooks key developments. The period between October 2024 and June 2025 was marked by ongoing attacks against Israel by the Houthis—a close Iranian ally, if not a proxy. These sustained assaults kept the risk of renewed conflict high.

More fundamentally, this interpretation rests on a flawed understanding of the applicable legal framework. The decisive issue is not whether a continuous exchange of force occurred between Israel and Iran during the period from October 2024 to June 2025, but whether Israel’s use of force in June 2025 conformed to the legal constraints applicable to an armed conflict that had commenced months earlier. So long as its conduct conformed to the core principles of international humanitarian law—including necessity, distinction, and proportionality—Israel’s actions were consistent with the requirements of the law of armed conflict.

Reframing Immediacy: A Function of Necessity, Not a Separate Threshold

The requirement of immediacy in the exercise of the jus ad bellum right of self-defense has long been a source of conceptual confusion. As Tom Ruys notes, the traditional interpretation of the immediacy requirement holds that a state must undertake its defensive response either contemporaneously with, or shortly following, the initiating armed attack. The rationale for this conclusion, however, is unclear and, upon closer scrutiny, unconvincing.

There are numerous legitimate reasons why a state might not respond immediately to an act of aggression. Diplomatic considerations may counsel delay in the hope of resolving the conflict peacefully. Delay may be justified by the need to gather actionable intelligence, obtain and deploy appropriate military equipment, mobilize forces, develop operational plans, secure internal political consensus, and coordinate with allies. The requirement of immediacy must be interpreted in light of the practical, strategic, and institutional demands inherent in mounting a lawful and proportionate military operation.

Indeed, treating immediacy as a strict requirement may perversely encourage the very escalation of violence international law seeks to prevent. If states believe their right to self-defense will lapse unless they act swiftly, they may feel pressured to launch premature or ill-considered military operations. This “use it or lose it” dynamic elevates legal timing over strategic judgment and may foreclose peaceful alternatives before they are meaningfully explored. In this way, the immediacy requirement may incentivize conflict rather than deter it.

The most common justification for an immediacy requirement is that it serves as a safeguard against abuse: By requiring a prompt response, it prevents states from invoking long-past acts of aggression as a pretext for military action that is not genuinely defensive. However, this concern is unpersuasive. By definition, a use of force that is necessary for self-defense cannot be deemed pretextual.

In light of these realities, it is misguided to treat delay as inherently suspect. A more principled approach is to evaluate whether the response—whenever it occurs—was necessary to protect the state from further harm. On this view, immediacy should be considered only to the extent that it relates to the broader criterion of necessity in self-defense. The central inquiry should be whether the use of force was necessary at the time it was undertaken—not whether it occurred soon after the initial attack.

This interpretation reflects a more pragmatic approach to the jus ad bellum principle of necessity. In assessing the lawfulness of a state’s use of force, state practice often places less emphasis on the timing of the response and focuses instead on whether the force employed was necessary and proportional to the threat confronted. In Nicaragua v. United States, the International Court of Justice (ICJ) found that the use of armed force several months after an attack on El Salvador indicated that further resort to force was no longer necessary.

Crucially, the ICJ’s reasoning rested not on the delayed nature of the response, but on the absence of continuing necessity at the time force was employed. Accordingly, it is both doctrinally sound and practically sensible to consider immediacy only to the extent it relates to necessity, as opposed to treating it as an independent bar to delayed defensive action.

This approach is also consistent with the widely accepted understanding of when an international armed conflict legally comes to an end. As articulated in the International Committee of the Red Cross (ICRC) Commentary (2020) to the Third Geneva Convention, an armed conflict continues to exist until “the likelihood of the resumption of hostilities can reasonably be discarded” (para. 311). This formulation reflects the view that the termination of an armed conflict is marked not by a cessation of active combat, but by the elimination of a credible threat that hostilities will resume. It thus reinforces the central thesis advanced here: that necessity is the governing criterion, for once hostilities have genuinely ended and the threat of their resumption has been definitively eliminated, there is no longer any need for a further resort to force.

The proper inquiry, therefore, is whether Israel’s use of force in June 2025 was necessary and proportionate in light of the prevailing threat environment at that time. If so, Israel in June 2025 retained the right to take defensive action against Iran’s 2024 missile attacks, irrespective of the intervening time period during which hostilities were paused.

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In sum, Israel’s use of force in June 2025 need not be justified under the narrow framework of anticipatory self-defense. The existence of an ongoing international armed conflict with Iran—triggered by Iran’s direct missile attacks in 2024—supports the view that Israel was already engaged in a lawful exercise of self-defense. As long as the threat from Iran persisted, Israel was entitled to respond, even after a prolonged operational pause. A rigid insistence on immediacy distorts the law of self-defense and undermines a state’s capacity to respond judiciously to enduring threats. The more appropriate legal test focuses not on the speed of the response, but on whether it was necessary and proportionate to counter the threat posed by Iran.