Jus ad Bellum Under Strain: The Illegality of the June 2025 Strikes and September’s Legal Risk Picture

[Winona Xu teaches international law and reproductive health at UCLA]

Introduction

As of September 15, 2025, eight weeks after Israel’s Operation Rising Lion (June 13) and the U.S. companion strikes (Operation Midnight Hammer, June 21–22) on Fordow, Natanz, and Isfahan, the immediate legal question has fused with an acute non‑proliferation and nuclear‑safety crisis. The IAEA still cannot verify the fate of Iran’s 60% enriched uranium stockpile (≈440.9 kg) because inspections at key sites have been blocked since the attacks; a tentative access deal was announced last week but remains conditional and vague on modalities. In parallel, the UK‑France‑Germany trio (the E3) have triggered the UN “snapback” process to restore sanctions, with signals they could pause it if Iran restores full inspection access and re‑engages in U.S. nuclear talks. Meanwhile, Washington now characterizes the U.S. strikes as having degraded Iran’s capacity but likely only for “months,” even as it publicly presses for complete dismantlement of enrichment and reprocessing in Iran. Regionally, Iran’s retaliatory missile launch toward the U.S. Al Udeid base in Qatar (intercepted with no casualities) underscored the ease with which the conflict can widen beyond the Israel–Iran exchange. This article revisits the UN Charter framework considering these developments, arguing that the June strikes remain unlawful uses of force and explains why today’s verification gap, sanctions dynamics, and cross‑domain escalation risks make adherence to jus ad bellum increasingly urgent.

From a strict legal perspective, Israel’s Operation Rising Lion and the United States’ subsequent use of force were manifest violations of the Charter’s core rules governing the resort to force. The UN Charter prohibits the threat or use of force by states except in two narrowly defined circumstances: with authorization from the UN Security Council, or in the exercise of self-defense in response to an actual armed attack. In this case, neither condition was met. Iran (and its affiliated proxies) had not launched any ongoing or imminent armed attack against Israel prior to the Israeli strikes on June 12–13. Consequently, Israel’s unilateral use of force – and the U.S. military intervention that followed – remains legally unjustified under Article 51 of the Charter or any other exception.

The UN Charter’s Framework: Articles 2(4) and 51

What’s Changed Since June — Legal Relevance at a Glance:

• The strikes destroyed or disabled safeguarded facilities and cut off inspection access, so the IAEA cannot account for 60% enriched uranium (≈440.9 kg) accumulated pre‑attack. This raises present‑day diversion and breakout risks, and makes any claims of “necessity” or “imminence” even more contestable in retrospect.
• Israel’s June 17 and the United States’ June 27 Article 51 letters to the UN now fix their legal justifications (anticipatory and collective self‑defense) for scrutiny. Both letters assert Iran’s aggression via nuclear ambitions and proxies, but critical commentary finds the U.S. letter materially deficient regarding imminence and the attribution of attacks to Iran’s control.
• Early U.S. intelligence assessments indicate the strikes likely delayed Iran’s nuclear capabilities by mere months, sharpening questions of proportionality vis‑à‑vis the humanitarian and systemic risks created.
• The E3 formally instated UN sanctions snapback on Aug. 28 (a 30‑day countdown). They have floated a pause if Iran restores full IAEA access and engages in talks. That legal and diplomatic recourse, now underway, was the Charter-compliant alternative that the June unilateral strikes impeded.
• The IAEA has also warned of degraded nuclear safety and security post‑attacks. No off‑site radiation release has been detected so far, but the June strikes set a dangerous precedent for armed attacks on safeguarded nuclear infrastructure during conflict.
•  Iran’s missile launch toward Al Udeid (intercepted) and continuing proxy threats keep the specter of collective self-defense claims alive, but none of these subsequent developments cures the illegality of the initial resort to force in June.

Article 2(4) of the UN Charter remains unequivocal: states shall refrain from the threat or use of force against the territorial integrity or political independence of any state. This fundamental rule, widely regarded as a peremptory norm, admits only a few narrow exceptions. The Charter entrusts the UN Security Council with the authority to authorize forceful measures when necessary to maintain or restore international peace and security. Separately, Article 51 preserves an inherent right of self-defense, but strictly “if an armed attack occurs” against a member state. In other words, aside from Security Council action, a state may resort to unilateral or collective force only in response to an armed attack that is already underway or at least imminent. Any use of force that falls outside these confines is illegal.

In the case of Operation Rising Lion, neither of these justifications existed. The UN Security Council did not authorize Israel’s strikes on Iran. Indeed, in an emergency session on June 13, multiple Security Council members condemned Israel’s attack as a clear violation of the UN Charter. Lacking Council approval, Israel’s only possible legal defense would be self-defense under Article 51 – yet that argument collapses upon scrutiny, because no Iranian armed attack on Israel had occurred or was imminent at the time Israel launched its bombardment. The well-settled rule under Article 51 (confirmed by the International Court of Justice in Armed Activities on the Territory of the Congo (DRC v. Uganda)) is that the right of self-defense arises only in response to an actual armed attack or, at most, an attack that is truly imminent. Israel’s situation does not meet this criterion. There was “no armed attack by Iran against Israel occurring on June 12, or about to occur”, as Professor Adil Ahmad Haque observed, “no ongoing armed attack, incipient armed attack, imminent armed attack, or impending armed attack” that could justify Israel’s use of force. By initiating a strike absent a prior or imminent enemy attack, Israel transgressed Article 2(4).

Under international law, the burden was on Israel (and later the U.S.) to demonstrate that their military actions fell within the narrow self-defense exception. This requires satisfying the well-known conditions of necessity and proportionality, and critically, the existence of an armed attack to respond to. As detailed, that factual predicate was absent here. Without an armed attack by Iran, Israel’s resort to force was, per se, illegal. The subsequent American strikes on Iran are likewise illegal, as the U.S. cannot independently justify its use of force except by reference to Israel’s self-defense (or a separate armed attack on the United States, which did not occur). In short, both states’ actions run afoul of the Charter’s essential rules restraining unilateral force.

By all accounts, Iran’s posture in early June 2025, while hostile rhetorically, was defensive within its own borders. Its leaders had indeed issued threats and inflammatory statements about destroying Israel. However, as one Professor Haque aptly notes, “vile statements” or hostile intent do not amount to an armed attack or give a legal green light for preventive war. International law requires tangible armed aggression, not merely aggressive words or speculative future dangers. If bellicose rhetoric alone justified force, history would be an unrelenting chain of devastating wars. In this case, notwithstanding harsh words from Tehran, there was zero indication of Iranian forces preparing to launch an actual strike on Israel in the hours or days before June 13. Israeli officials themselves described Operation Rising Lion as aimed at preempting what they called an “existential” threat – namely, Iran’s potential acquisition of nuclear weapon capability in the future. That is a far cry from repelling an actual attack.

Even Iran’s support for militant proxies (Hamas, Hezbollah, Houthis, etc.) did not translate into any ongoing armed attack against Israel in mid-2025. Those groups’ attacks on Israel, while serious, were separate incidents and had diminished at that time. Israel’s strikes were not directed at stopping an active proxy attack (indeed, Israel struck deep into Iran’s territory, not proxy bases), undermining any claim that it was halting an incipient assault. In its belated letter to the Security Council on June 17, Israel vaguely referenced “Iranian missile and proxy attacks” as an imminent threat, and Iran’s support for armed groups. But Israel’s use of force was not actually focused on those militias or any specific attack unfolding; it was focused on Iran’s own nuclear and military infrastructure. Thus, the precondition for lawful self-defense – an armed attack (actual or truly imminent) – simply did not exist. Israel’s operation violated Article 2(4) from the outset by striking another state absent the requisite trigger.

Lack of Security Council Authorization

Compounding the illegality of Israel’s and the U.S.’s actions was the total absence of approval from the UN Security Council. Under the collective security system established by the Charter, the Council is the only body empowered to authorize military force aside from self-defense situations. In scenarios that do not involve an actual armed attack, a state concerned about threats (even serious ones like Weapons of Mass Destruction (WMD) proliferation) must bring its case to the Security Council and seek a Chapter VII resolution. Unilateral action is not permitted. Here, Israel chose to bypass the Security Council. In fact, Israel did not even notify the Council under Article 51 immediately after launching its strikes – a telling omission that indicates Israel knew it was not responding to an “armed attack” in the Charter sense. (Israel eventually sent a letter on June 17, once the operation was well underway, framing its actions in defensive terms but still not explicitly invoking Article 51.) When convened, many Council members condemned it, and none endorsed it. No resolution authorizing force against Iran was even seriously proposed. Therefore, the only possible legal cover for Israel or the United States was self-defense – which, as has been shown, did not apply.

The United States similarly did not seek Security Council authorization for its own strikes on Iranian targets. American officials justified their involvement as aiding an ally under attack (Israel) and protecting regional security, but these arguments were never presented to or approved by the UN collective security framework. The U.S. acting on its own accord militarily against Iran, without Iran having attacked the U.S. and without UN approval, plainly contravenes Article 2(4). Washington’s actions, in effect, amount to joining one side of an unlawful conflict without international sanction. The lack of Security Council authorization is especially glaring given the implications of a major power attacking Iran’s territory – a situation the Council was attempting to address diplomatically. By circumventing the UN and taking matters into their own hands, Israel and the United States undermined the very system meant to handle threats to peace cooperatively. This sets a dangerous precedent: if any nation could unilaterally attack another because it suspects a future threat, and then other allies join in, the Charter’s collective security architecture would be rendered meaningless.

Jus ad Bellum vs. Jus in Bello: Ongoing Conflict Does Not Legalize New Strikes

Some commentators have attempted to argue that Israel’s operation might be lawful by virtue of occurring within an existing armed conflict between Israel and Iran, rather than as a new use of force requiring separate justification. According to this view, Israel and Iran were already in a de facto international armed conflict (due to Iran’s support of groups fighting Israel and prior direct exchanges), and therefore Israel’s June 13 strikes were simply an escalation in an ongoing war, governed solely by the law of armed conflict (jus in bello) rather than the law of self-defense. Proponents of this theory, drawing on the late Professor Yoram Dinstein’s scholarship, contend that once an international armed conflict is underway, a state may continue military operations until the enemy’s capacity to fight is eliminated, without needing to demonstrate a new necessity for each action. In other words, they suggest that the Charter’s jus ad bellum constraints (like the need for an armed attack or imminence) cease to apply once hostilities have begun; only jus in bello (e.g. targeting and humanitarian rules) would regulate the conduct of the war going forward.

This argument is fundamentally flawed. Jus ad bellum and jus in bello are distinct and complementary bodies of law, and an ongoing conflict does not give states a free hand to expand the scope or scale of force without restraint. The mere existence of hostilities does not nullify Article 2(4) or suspend the requirement that any use of force be justified under the Charter. To be sure, if two states are already at war, not every minor skirmish or tactical move will be individually assessed under jus ad bellum as a “new” armed attack – the conflict may be viewed as the armed attack that triggered initial self-defense rights. But when there is a significant escalation or a new front opened in a conflict, international law demands a fresh look at whether that action adheres to self-defense parameters. As Professor Schmitt notes, “so long as future operations are reasonably foreseeable and hostilities have been relatively uninterrupted, there is a continuing right of self-defense.” However, “when a significant shift in the nature of the conflict occurs, as is the case with the targeting of Iran’s nuclear assets following a relative lull in the fighting, it is appropriate to reassess whether there is a basis for that particular use of force and whether it complies with the conditions of proportionality and necessity.” In other words, launching a major new offensive (like Israel bombing Iran’s nuclear facilities after months of quiet) is not automatically covered by whatever initial casus belli might have existed in the past; it must be justified on its own merits under Article 51.

Furthermore, conflating jus ad bellum with jus in bello is dangerous. Jus in bello governs how war is conducted – protecting civilians, banning certain weapons, etc. It applies regardless of whether a war is lawful or unlawful in origin. Thus, even if Israel’s campaign is illegal under jus ad bellum, both Israel and Iran must still comply with international humanitarian law (IHL) in how they fight (a point addressed later). But the reverse is not true: compliance with IHL or the mere fact of being in a state of armed conflict does not automatically satisfy jus ad bellum. A war can be fought in accordance with the laws of warfare and yet still be an unlawful war of aggression. Here, scholars arguing that Israel was already engaged in an ongoing international armed conflict with Iran, so Israel didn’t need a new Article 51 justification for expanding attacks points to a misinterpretation of the law. Even during an ongoing conflict, if one party undertakes a major escalation, it must ensure that action is defensible as necessary and proportionate in response to enemy conduct. In practical terms, Israel still needed to point to an Iranian armed attack or a new immediate threat in June 2025 to justify its strikes – and it could not. As Schmitt writes, acknowledging an ongoing conflict “does not detract from the factual reality” of hostilities, but “for policy reasons, there may be hesitancy in acknowledging [it].” Crucially, even he concludes that if there was a “relative lull” and then a significant change like targeting nuclear assets, one should reassess the jus ad bellum basis.

State practice and opinio juris continues to reject the notion of a perpetual, open-ended right to use force once conflict starts. The better view – and the correct statement of law – is that jus ad bellum limits continue to apply at all times. In this case, those limits were breached. The ongoing Israel-Iran hostilities do not whitewash the illegality of a sudden preventive strike on Iran’s nuclear program. The “war context” argument is essentially an ex post facto excuse to avoid the Charter’s requirements, and it must be firmly rebutted. The Charter does not cease to function whenever states are at war; on the contrary, it was designed precisely to regulate and limit war, even wars in progress, to prevent escalation and wider breaches of peace. The Article 51 filings have since clarified the parties’ legal theories. Israel’s June 17 letter to the UN leaned on an asserted “last resort” necessity to thwart supposedly imminent attacks, and the U.S.’s June 27 letter claimed collective self-defense on behalf of Israel, pointing to Iran’s nuclear capacity and its arming of proxies. These formal justifications have been widely critiqued for not meeting the high threshold of imminence or necessity required. That public record of justification hardens, rather than mitigates, the jus ad bellum concerns discussed above.

The United States’ Use of Force: Collective Self-defense or Unlawful Intervention?

On June 23, Iran launched a missile toward Al Udeid in Qatar. It was intercepted with no casualties, material to the scale and effects threshold under Nicaragua. Even if that incident independently supported narrow individual self‑defense for immediate point‑defense, it cannot retroactively legalize the U.S. June 21–22 offensive strikes inside Iran. For collective self-defense to be lawfully invoked, two key conditions must be met: (1) the state requesting assistance (here, Israel) must itself have been the victim of an armed attack, and (2) that state must expressly request or consent to the aid. If we assume Israel “requested” U.S. help once Iran began retaliating with missile strikes, thus the pivotal question is: was Israel truly the victim of an armed attack by Iran, within the meaning of international law? If Iran’s missile salvos after June 13 are viewed in isolation, they certainly were armed attacks in a descriptive sense – missiles struck Israeli cities and bases, killing and wounding people. But legally, can Iran’s response be characterized as an unlawful armed attack, given that it was provoked by Israel’s own wrongful use of force? There is an argument that Iran’s counterstrikes, however deadly, were not an “armed attack” triggering Israel’s (or the U.S.’s) self-defense rights, but rather a lawful response by Iran in self-defense to Israel’s aggression. In international law, an aggressor state cannot readily cloak itself in self-defense if the victim of its aggression retaliates. Allowing that would invert the law: the original lawbreaker would benefit from its wrongdoing by claiming self-defense against the consequences of its own illegal act.

The U.S. might counter that regardless of the initial legality, once Iranian missiles started falling on Israeli cities (some reportedly hitting civilian apartments and a hospital), those strikes should be seen as unlawful in themselves (due to targeting civilians) and thus qualify as armed attacks. It is true that any intentional attack on civilians violates international humanitarian law and cannot be justified as self-defense necessity. If Iran deliberately targeted civilian objects, those specific actions are war crimes and fall outside legitimate self-defense, as self-defense does not permit attacking civilians. In theory, one could argue that even if Iran had a right to respond to Israel, it had to do so within IHL limits; that by hitting civilians, Iran committed armed attacks of its own, triggering Israel’s (and its allies’) right to defend against those unlawful attacks. However, it is critical that this nuanced argument does not mix jus ad bellum and jus in bello considerations. It suggests that amid an ongoing conflict, if one side commits egregious IHL violations (like bombing cities), the other side can invoke self-defense to prevent those violations.

Even accepting that reasoning for the sake of argument, the U.S. actions still appear excessive and unauthorized. Intercepting incoming missiles to protect Israeli civilians is one matter – arguably a defensive, necessary act to prevent harm. But launching U.S. strikes on Iranian soil, bombing Iran’s nuclear facilities or military installations, with civilian casualties reported at over 430 lives, goes beyond pure defense. Such actions enter the realm of taking the offensive. The U.S. strikes did not directly stop Iranian missiles already in the air; they were aimed at degrading Iran’s capacity, potentially to discourage further attacks or to aid Israel’s broader campaign. Such offensive operations are hard to justify as “collective self-defense” limited to repelling an attack. The proportionality of the U.S. response is highly dubious. Moreover, from a U.S. perspective, Iran had not attacked the United States. American forces were not under Iranian attack (unless Iran had incidentally struck U.S. assets, which has not been reported). Therefore, the U.S. cannot claim individual self-defense. Its only recourse was to claim it acted on Israel’s behalf – which brings back Israel’s lack of claim to lawful self-defense in the first place.

UN Charter Obligations Against Unilateral Force

It is also important to highlight that no bilateral security agreement or alliance can override the UN Charter’s prohibitions. The U.S. and Israel have a very close strategic relationship, but their various defense agreements do not contain an Article 5-style mutual defense clause requiring automatic military intervention. And even if they did, treaty obligations cannot excuse a breach of the Charter. Article 103 of the UN Charter makes clear that obligations under the Charter prevail over any other international agreement. Thus, the U.S.’s professed commitment to Israel’s security does not create a legal loophole to use force against Iran absent a valid Charter basis. Washington, like all UN members, is bound by Article 2(4) and may only use force in self-defense or with Security Council approval. Here, it had neither.

The United States’ direct involvement in hostilities against Iran stands on very shaky legal ground. By intervening in support of what was (from the outset) an unlawful Israeli attack, the U.S. essentially became a co-participant in that illegal use of force. At best, U.S. officials could argue they were defending Israel against Iran’s counterstrikes, but since those arose only because of Israel’s initial wrongdoing, this justification rings hollow. A more sound legal assessment is that the U.S. use of force, lacking an independent trigger or UN mandate, violates the UN Charter just as Israel’s does. It illustrates how one state’s unlawful resort to force can pull others into the breach, threatening to unravel the collective security framework. Such cascade effects underscore why the Charter takes a hard line against unilateral force: to prevent conflicts from spiraling in exactly this way.

Conclusion

The June 2025 strikes by Israel and the United States remain manifest violations of the UN Charter. What is new, eight weeks on, is the perilous risk surface they have created: an IAEA blind spot over a large stockpile of high-enriched uranium, a looming snapback sanctions cliff, and the normalization of attacking nuclear infrastructure — a precedent other states may be tempted to cite tomorrow. Even U.S. intelligence admits the military gains are likely temporary, while the legal and institutional damage is durable. The Charter’s constraints on the use of force are not mere formalism; they are functional safeguards against exactly this kind of strategic backfire. Today’s situation demonstrates that upholding those rules is more essential than ever, lest we suffer the lasting consequences of a world where might overrides right.