On the night of June 12, 2025, the Israel Defense Forces (IDF) began an ongoing campaign of airstrikes across Iran, bombing military facilities, nuclear sites, civilian industrial sites, and civilian apartment buildings. An IDF airstrike on one apartment building reportedly killed 20 children. By June 19, IDF airstrikes had killed at least 263 civilians and wounded at least 335 civilians, according to one widely-cited Iranian NGO. Israel calls its military campaign “Operation Rising Lion.” Iran has responded with ballistic missile attacks striking military facilities, civilian apartment buildings, and at least one hospital. Both Israel and Iran are violating international humanitarian law and must stop. This essay will focus on the legal status of Israel’s military offensive under the UN Charter.
Israel’s use of force against Iran was and remains plainly unlawful and a manifest violation of the United Nations Charter. The Charter prohibits the use of force except with the authorization of the UN Security Council or in the exercise of the right of self-defense “if an armed attack occurs.” No armed attack by Iran against Israel was occurring on June 12, or was about to occur, or was bound to occur unless prevented by the immediate use of force. There was no ongoing armed attack, incipient armed attack, imminent armed attack, or impending armed attack. The law is more than well-settled. When Israel attacked Iraq’s nuclear facility in 1981, the United Nations Security Council unanimously adopted a resolution that “[s]trongly condemn[ed] the military attack by Israel in clear violation of the Charter of the United Nations” and “[c]all[ed] upon Israel to refrain in the future from any such acts.”
The UN Security Council did not authorize Israel’s use of force against Iran. At an emergency meeting, several Council members declared it illegal. Other members called for de-escalation and a return to diplomacy, logically implying that Israel’s use of force is not necessary to prevent an armed attack by Iran. As the International Atomic Energy Agency found, Iran’s failures to co-operate fully with the Agency “gives rise to questions that are within the competence of the United Nations Security Council, as the organ bearing the main responsibility for the maintenance of international peace and security.” Iran’s failures do not legally justify the unilateral use of armed force. The patent illegality of Israel’s use of force was promptly but patiently explained in an earlier essay by Marko Milanovic.
On June 17, Israel sent a letter to the United Nations Security Council informing the Council of its ongoing military operation. The letter does not expressly invoke Israel’s right of self-defense under international law or mention article 51 of the UN Charter (which recognizes that right). Nevertheless, the letter says various things that sound like international law. Israel claims to be “acting to defend its security and very existence” against “Iran’s ongoing aggression.” It refers to an “existential and imminent threat from Iran’s nuclear weapon and ballistic missile program” as well as the “imminent threat” of “Iranian missile and proxy attacks.” It describes its military operation as “a last resort” taken in “the last window of opportunity to prevent Iran from acquiring nuclear weapons … after diplomacy proved ineffective.” It also says that Iran is “substantially involved” in “persistent and unlawful attacks” by Hezbollah, the Houthis, Hamas, and “others.”
While Israel’s letter sounds like international law, it is not international law. Is Israel saying that an armed attack by Iran was imminent? There was no such imminent armed attack, let alone an attack using nuclear weapons that Iran does not possess and has not decided to pursue (according to U.S. intelligence). Is Israel saying that an armed attack by armed groups that Iran supports was imminent? Israel’s use of force was not directed at these armed groups or at cutting off Iran’s support for them as self-defense rules of proportionality and necessity would require.
Israel offers no evidence of an imminent armed attack. It cites a statement by Iran’s Supreme Leader Ayatollah Ali Khamenei that Israel is “a cancerous tumor that must be eradicated and it will be.” Set aside the conspicuous use of the passive voice and the omission of any reference to nuclear weapons or military action. If vile statements could justify the use of force then humanity would never have survived the Cold War. (Those judging Khamenei by his words should note that he has consistently said, over many years, that Iran does not want nuclear weapons and considers their use contrary to Islam). Israel recalls Iran’s direct attacks on Israel in April and October 2024. Israel neglects to mention that these were discrete (unlawful) armed reprisals responding to specific Israeli operations, not indications of future attacks. Finally, Israel refers to attacks on Israel by various armed groups which Iran supports. Set aside whether these amount to armed attacks by Iran or armed attacks attributable to Iran under the law of State responsibility. Israel’s use of force is not directed against these groups or against Iran’s lines of support for them. Missiles launched by the Houthi from Yemen cannot legally justify bombing nuclear energy facilities in Iran.
In an earlier essay, one legal scholar opined that it was “not unreasonable” for Israel to conclude that Iran was “irrevocably committed” to building a nuclear weapon and using it in an armed attack against Israel, and that such an irrevocable commitment might justify the use of force despite the absence of a “temporally” imminent attack in the near future. This conclusion is, in fact, unreasonable. There is no substantial evidence to support it. It defies common sense. Let us speak plainly. The single most important fact about Israel and Iran is that Israel has dozens of nuclear weapons and Iran has none. Iran does not intend to attack Israel with nuclear weapons (which it does not have and, according to the U.S. intelligence community, has not decided to build). Israel would retaliate in kind and Iran would be incinerated, its people killed, its heritage erased, and its form of government wiped from the Earth. Every credible expert on Iran’s national security strategy agrees that its overriding aim is the survival of the regime and the nation. Any notion that Iran intends to trigger mutually assured destruction is delusional and cannot provide a rational basis for the use of force under international law.
Armed Attack and Armed Conflict
On June 18, Israel’s Deputy Attorney General wrote that Israel’s military offensive “was launched as part of the ongoing armed conflict between [Iran] and Israel, in light of the existential and imminent threat from Iran’s nuclear weapon and ballistic missile programs.” He went on to write:
Iran has engaged in an armed conflict with Israel for years, directly as well as indirectly, through various proxies in the middle east. One recent peak of this ongoing armed conflict was marked by Iran’s missiles and UAV attacks on Israel during April and October of 2024, historically unprecedented in their scope. Accordingly, Israel’s actions should be assessed in light of the law of armed conflict.
Two legal scholars appear to make similar claims in an earlier essay (though their intended meaning is not always clear). They concede that if Israel’s military operation “involves a use of force in anticipation of a non-imminent future threat—nuclear or conventional— there is good reason to deem it unlawful.” However, they appear to argue, at least in part, that Israel’s military operation was not a resort to force but an escalation of an ongoing “armed conflict” within which preemptive action is legally permissible.
Such claims make no sense. They conflate two bodies of international law. Under international law, an “armed conflict” triggers the application of the law of armed conflict (also known as international humanitarian law or IHL) to both sides of the conflict. But an “armed conflict” does not authorize the use of force by either side of the conflict, let alone by both sides. Logically, if the existence of an ongoing armed conflict provided an independent legal basis for the use of force, then it would follow that Iran was legally permitted to attack Israel before June 12. It was not. Israel was not permitted to attack Iran either.
Only an “armed attack” within the meaning of the UN Charter can authorize one State to use force against another in self-defense. While the terms “armed conflict” and “armed attack” sound similar, they mean different things and serve different purposes. The term “armed conflict” may be interpreted broadly—including to apply during long lulls in active hostilities—since its legal effect is to protect persons and constrain military operations. To illustrate with a different example, the detention of a single soldier may suffice to trigger an “international armed conflict” and with it the protections afforded by the Geneva Conventions, but would not amount to an “armed attack” triggering the right of self-defense. In contrast, the term “armed attack” must be interpreted narrowly to ensure that force is not authorized when there is no ongoing or imminent attack that it is necessary to halt or repel by force. Self-defense is a limited exception to the general rule prohibiting force, and “armed attack” must be narrowly construed lest the exception swallow the rule.
The fundamental point is that even if there were an ongoing armed conflict between Israel and Iran (alone or along with the armed groups it supports), this would not show that there was an ongoing or imminent armed attack necessitating the use of force against Iran. There was none. While “Israel’s actions should be assessed in light of the law of armed conflict” they must also be assessed in light of the UN Charter, which they manifestly violate.
According to the ICRC, an international armed conflict may persist through long lulls in hostilities until the “general close of military operations” clearly indicating the “final end of all fighting between all those concerned.” Even “redeploying troops along the border to build up military capacity or mobilizing or deploying troops for defensive or offensive purposes” may extend an armed conflict for months or even years despite no exchanges of fire. Does this mean that one or both States may resume the use of force against the other anytime they wish? Of course not. Frozen conflicts should stay frozen. Ceasefires should lead to lasting peace. Lulls in fighting should be opportunities to settle disputes by peaceful means or invite the UN Security Council to determine the existence of (non-imminent) threats to the peace and take measures to maintain international peace and security. The persistence of an armed conflict extends the protections of IHL. It does not indefinitely extend the permission to use force in self-defense.
Nothing changes if Israel is a party to an ongoing armed conflict with Iran and various armed groups it supports, notably Hamas, Hezbollah, the Houthis. For such a single “internationalized” armed conflict to exist, Iran would have to exercise either “overall control” or “effective control” over these armed groups that goes beyond financing and equipping them (for more on this doctrinal debate see here). This seems doubtful, but it is also beside the point. All that would follow from the existence of such an internationalized armed conflict is that the law of international armed conflict would apply to future hostilities between Israel and these armed groups. Nothing would follow about whether Israel may use force in self-defense in Iran that does not aim to halt, repel, or preempt an imminent armed attack.
Finally, Israel’s letter claims that Iran “has been, and continues to be, substantially involved in their proxies’ persistent and unlawful attacks against Israel.” Under international law, a State’s “substantial involvement” in sufficiently grave acts of armed force by armed groups may constitute an act of aggression and an armed attack by that State. The International Court of Justice has held that “substantial involvement” requires more than “assistance … in the form of the provision of weapons or logistical or other support” but the exact threshold is unclear. It is also irrelevant to the question at hand. Israel’s use of force was not directed at these armed groups or at Iran’s lines of support for them, so it would fall outside the legal limits of necessity and proportionality internal to the right of self-defense. Any attempt to circumvent the limits of the right of self-defense by conflating it with the law of armed conflict must be rejected. These distinct bodies of law perform different functions, and this attempt at conflation would undermine both.
Conclusion
Israel’s use of force against Iran was and remains a manifest violation of the UN Charter. It was neither authorized by the UN Security Council nor a response to an actual or imminent armed attack by Iran. It is that simple.
At the time of this writing, the President of the United States is reportedly considering joining Israel’s military offensive against Iran. Doing so would involve the United States in an unlawful act of aggression. It would not be the first time.
Editor’s note: For a different perspective, see Amichai Cohen and Yuval Shany, A New War or a New Stage in Ongoing War – Observations on June 13 Israeli Attack against Iran (June 15, 2025)
