Israel’s Rising Lion

It is never trite to emphasize that the law on the use of force is straightforward: it prohibits the use of force in international relations. Before 1945, it was left largely to states themselves to judge necessity and legitimacy, resulting in a much broader and more subjective scope for unilateral action. The adoption of the UN Charter marked a sea change.

Today, jus ad bellum endures only as a narrow exception within the prevailing jus contra bellum framework. In short, the legal order built after World War II deliberately made the use of force the exception, not the rule. This architecture is intentionally restrictive; self-defense and collective enforcement exist only as tightly defined fallbacks, each confined within and subordinate to the overall prohibition on force.

Yet, states are increasingly testing the outer limits of the self-defense exception. In June 2025, Israel’s operation “Rising Lion”, a bid to prevent a future nuclear threat rather than to repel an actual or imminent attack, has been described by some as a case of preventive self-defense, a logic that does not sit with the Charter’s restrictive framework (see here, ch. 7). Under vastly different circumstances and on a much larger scale, Russia cited a similar rationale for its 2022 invasion of Ukraine, invoking the language of self-defense against hypothetical dangers. Although the two cases differ dramatically in context and scale, both illustrate how the self-defense exception is being stretched and how the lines between lawful defense and unlawful force are growing ever more contested.

Jus ad Bellum as an Exception

To understand just how far the legal framework is being stretched, it is important to distinguish between three key concepts: jus contra bellum, jus ad bellum, and what might be called jus ad bellum within jus contra bellum. As outlined above, jus contra bellum, literally, the law against war, is enshrined in Article 2(4) of the Charter, making clear that the use of force is not a standing entitlement but an absolute ban.

By contrast, jus ad bellum refers to the body of law governing when states are permitted to resort to armed conflict. Under the Charter system, jus ad bellum is a tightly circumscribed set of exceptions that operate within jus contra bellum. The use of force is prohibited unless it fits one of these two narrow gateways: (1) individual or collective self-defense, as permitted by Article 51; or (2) enforcement measures authorized by the Security Council under Chapter VII. These exceptions are not open-ended but are bound by the Charter’s requirements of necessity, proportionality, and prompt reporting.

The right of self-defense under Article 51 of the UN Charter has been interpreted along three main lines, each setting a distinct threshold for the lawful use of force. First, the most restrictive understanding allows self-defense only in response to an actual armed attack. It is often considered too rigid. In practice, few States would sit idly by as a grave threat materializes, waiting to absorb the first blow.

Second, under an anticipatory or imminent threat approach, self-defense may be invoked when an attack is not yet underway but is clearly about to occur. Even in such cases, any use of force must satisfy the additional requirements of necessity and proportionality, as consistently affirmed by the International Court of Justice (ICJ) (paras. 194, 237; para. 41; paras. 73-74, 76). This approach is now widely recognized as the governing standard in international law (para. 188).

Third, a preventive approach is associated with what became known as the “Bush doctrine.” The U.S. Bush administration invoked arguments justifying the use of force against Iraq based on the risk that Iraq might transfer weapons of mass destruction to terrorist actors, who could then use them against the United States. The United States assessed that the gravity of the threat warranted preventive action.

Imminence and the Rising Lion

Any assessment of self-defense must begin by asking whether the act a state seeks to forestall would constitute an “armed attack” within the meaning of international law (para. 195). There can be no doubt that a nuclear strike against Israel would meet the threshold of force required. The question, however, is whether there was persuasive evidence that such an attack was about to occur.

For present purposes, it is useful to distinguish between two theories of imminence. Under the first, an attack is imminent where preparations have reached an advanced, observable stage (para. 124; articles 3, 6). Under the second, an attack is imminent when a State has both the capability to conduct the attack and an intention, irrevocably made, to do so at some future point. This approach is often combined with the idea that now is the “last possible window of opportunity” to act (pp. 104-08).

Iran does not possess nuclear weapons and was not credibly shown to be preparing a direct military strike. Thus, Israel’s only plausible legal theory would be the causal, non-temporal one. But even here, two elements would have to be satisfied. First, Iran’s leadership must have irrevocably committed to attacking Israel with a nuclear weapon once that capability is achieved; that is, a specific intent to attack must be established. Second, the Israeli June 2025 strikes must have represented the last feasible opportunity to stop such an attack, with no viable, non-forcible alternatives remaining.

No concrete evidence has been offered that Iran’s leadership was committed to launching a nuclear attack on Israel. As far as de lege lata is concerned, Israel’s operation was not self-defense under Article 51. There is no publicly available evidence that Israel’s “last window” to preempt an armed attack by Iran had arrived. Instead, it represents an example of a preventive (unlawful) use of force.

Normalizing the Preventive Use of Force?

Statements from the United States, Australia, Germany, and France have offered explicit or implicit support for Israel’s resort to “self-defense,” without clarifying its legal basis. These endorsements may signal a worrying shift in the international discourse, potentially setting a precedent in which preventive use of force is increasingly normalized and accepted.

On 21 June, the United States (U.S.)  launched strikes on Iranian nuclear facilities, describing its actions as necessary to prevent an imminent threat and to support Israel’s right of self-defense. At first glance, the U.S. rationale mirrored Israel’s own: both framed their use of force as justified by the need to avert an attack that, if left unaddressed, might prove catastrophic. But the legal test is not whether a threat is serious or hypothetical; it is whether the circumstances satisfy the strict requirements of the Charter system. For the United States to lawfully invoke collective self-defense, several conditions must be met. First, collective self-defense presupposes that the state being assisted, in this case, Israel, has itself been the victim of an actual or imminent armed attack. There must be credible evidence that Israel faced such a threat, not just at some point in the distant future, but in the present or immediate term.

Second, the assisting state must make its own, independent legal assessment of the situation. It cannot simply rely on the requesting state’s assertion of threat; it must be convinced, on objective grounds, that the threshold for self-defense has truly been crossed.

The U.S. legal and policy analysis at this moment mattered profoundly. In part, this is because it was the United States that set the preventive use of force concept in motion in the early 2000s, giving it global prominence and controversy. But even more, the world’s attention was focused on Washington because the United States is the only actor with both the leverage and credibility to decisively influence Israel’s actions and policies regarding the use of force. The precedent set here will be closely watched, not just in the region, but by states everywhere contemplating how far, and under what conditions, they too might reach for the language of self-defense to justify preventive action.

As with Israel’s initial strike, the facts do not support the claim that an armed attack, actual or imminent, was underway or unavoidable. There is no public evidence that Iran had initiated, or was about to initiate, a qualifying attack on Israel. Accordingly, the U.S. strikes cannot be justified as lawful collective self-defense under Article 51. They represent another instance of preventive logic, the use of force against a speculative danger.

Conclusion

Israel’s Operation Rising Lion and the U.S. decision to join in have not only tested the edges of self-defense under international law but may well be setting a new direction for the law itself. What is at stake is not simply the fate of Article 51 or the credibility of the Charter framework, but the very idea of a rules-based order for international security. If the use of force can be justified based on potential or speculative future threats, then the architecture of jus contra bellum (not a perfect one) begins to erode. In its place emerges a looser, more permissive regime: jus ad bellum untethered from a rule-based objectivist constraint.

Once the boundaries of lawful self-defense are stretched or ignored by those with the power to do so, other states are sure to take note. The logic of preventive action may prove contagious, especially in regions where unresolved conflicts and deep-seated suspicions persist. In such a world, the subjective “upper hand” prevails; force becomes an instrument to be used “just in case,” to ensure one’s security, even at the expense of the broader system of mutual restraint that has, for decades, underpinned relative international stability. But can any state truly be safe in a world governed by such logic?

A devastating example is Russia’s invasion of Ukraine. Russia claimed it was acting to preempt an imminent Ukrainian attack. The only way to rationalize Russia’s position is through a preventive logic that treats vague, future potential threats as grounds for war.

As the ICJ aptly stated in the Armed Activities case (para. 148):

“Article 51 of the Charter may justify a use of force in self-defence only within the strict confines there laid down. It does not allow the use of force by a State to protect perceived security interests beyond these parameters.”

The international community must now decide: Will it reaffirm the Charter’s demand for clear, objective limits on the use of force? Or will it accept a world in which subjective assessments and shifting alliances set the rules?