In a letter to the U.N. Security Council dated March 10, 2026, the United States presented its international legal justification for the military campaign against Iran that it initiated on Feb. 28, 2026—Operation Epic Fury. The U.S. attack on Iran follows the June 2025 U.S. strikes on Iran’s nuclear facilities—Operation Midnight Hammer—which President Donald Trump claimed “obliterated” them.
Having helped draft such communiques on the use of force to the Security Council, I find the international law argument advanced by the United States in this so-called “Article 51 letter” unconvincing.
Both the U.N. Charter and the U.S. Constitution sharply limit recourse to force for very good reasons, under international and domestic law. It is incumbent upon both senior administration lawyers and the president’s top advisors to communicate this to him—as challenging as that might be (as I wrote in my assessment of the administration’s prior justification for the June 2025 attack on Iran).
Legal Background
Article 2(4) of the U.N. Charter prohibits “the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations.”
Article 51 of the Charter in turn specifies, in relevant part:
Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security. Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council
Therefore, notwithstanding the prohibition on the use of force imposed by Article 2(4), the use of forces in individual or “collective self-defense” (coming to the aid of another country exercising its right of self-defense) are permissible and must be reported to the U.N. Security Council. (The Security Council can also authorize the use of force under Chapter VII of the U.N. Charter.)
As I wrote last July:
Importantly, the text of the Article 51 requires an “armed attack” as a prerequisite for the use of force in self-defense. Although the United States and other States have endorsed the concept of anticipatory self-defense in response to an imminent threat of armed attack, some policy experts have advocated for States being able to use force in preventative self-defense against more inchoate threats. Consistent with the weight of State opinio juris and expert opinion, the U.S. government has never adopted this theory of preventative self-defense.
Indeed, in 1981, the United States joined a unanimous UN Security Council resolution in condemning a preventative Israeli attack on an Iraqi nuclear facility at Osirak. In the view of the United States, Israel’s actions violated Article 2(4) due to the “absence of any evidence that Iraq had launched or was planning to launch an attack that could justify Israel’s use of force. … [T]he presence in a State of the military capacity to injure or even to destroy another State cannot itself be considered a sufficient basis for the defensive use of force.”
Moreover, despite some misunderstanding to the contrary, the United States decidedly did not rely on preventative self-defense as the international law justification for the 2003 invasion of Iraq, but instead claimed authority to invade Iraq on the basis that Iraq materially breached a Security Council ceasefire resolution.
Any use of force in self-defense is also subject to the customary international law requirements of necessity and proportionality. For the use of force in self-defense to be lawful, it must be necessary. As articulated by the State Department’s Legal Adviser in 2016, the “international law of self-defense requires that such uses of force be necessary to address the threat giving rise to the right to use force in the first place.” As captured in the Caroline correspondence between the United States and the United Kingdom cited in that same speech, which articulates fundamental principles of self-defense, the “necessity of self-defence” requires “no choice of means.” Therefore, in order for the use of force to be necessary, peaceful alternatives to addressing the supposed threat—including diplomacy—must be unavailable or exhausted.
Conflating Jus in Bello and Jus ad Bellum
In the opening paragraph of the March 10th letter, the Trump administration asserts that U.S. combat operations against Iran were undertaken in the context of “the ongoing international armed conflict” that the United States previously addressed in its June 2025 letter regarding Operation Midnight Hammer. The invocation of a supposedly ongoing armed conflict is an apparent reference to a theory associated with the late Israeli scholar Yoram Dinstein, who wrote that “[t]he condition of necessity does not stand in the way of waging a war of self-defence until the enemy is utterly crushed and no longer poses an effective military menace,” and that “[o]nce war is raging, the exercise of self-defense may bring about the destruction of the enemy’s army regardless of the conditions of proportionality.”
But this ongoing armed conflict theory conflates two distinct bodies of law—jus in bello (which regulates the conduct of hostilities during armed conflict) and jus ad bellum (which regulates when States may resort to force). Whether an armed conflict exists as a factual matter such that the law of armed conflict applies has no bearing on whether the resort to force in the first instance was lawful.
For example, Russia has been very much a party to an international armed conflict with Ukraine since 2014. But the fact of that ongoing armed conflict does not alter the fact that Russia’s attack on Ukraine violated the U.N. Charter and was manifestly unlawful. Nor does that alter the reality that support by third States (like North Korea) to Russia’s aggression against Ukraine is also unlawful.
Despite the attempt to conflate jus in bello and jus ad bellum by invoking a supposedly ongoing international armed conflict, there were no ongoing hostilities between the United States and Iran prior to the Trump administration launching a fresh attack upon Iran with a new label, Operation Epic Fury. Indeed, according to a statement by the Secretary of Defense from June 2025, the prior U.S. attack on Iran’s nuclear facilities “was a resounding success, resulting in a cease-fire agreement and the end of the 12-day war.” To the contrary, the Feb. 28 attack represented a fresh recourse to the use of military force and was thus accompanied by both a new notification to Congress under the War Powers Resolution and a fresh Article 51 letter to the U.N. Security Council. Likewise, (at least according to the Trump administration) Israel was party to the ceasefire ending the 12-day war in summer 2025.
Collective Self-Defense?
The Trump administration next claims that its combat operations against Iran are being undertaken “in the collective self-defense of Israel.” What I wrote last July regarding a similar U.S. justification for Operation Midnight Hammer is true of this new missive as well. “[T]he letter fails to specify what supposed armed attack by Iran on Israel provided the predicate for the use of force in collective self-defense.”
Reflecting the reality that there was no prior armed attack by Iran against Israel, the next paragraph of the U.S. letter cites attacks by various members of the so-called “Axis of Resistance”—groups such as Hamas, Hezbollah, the Houthis, and paramilitaries in Iraq and Syria who have received varying amounts of Iranian support. But the letter does not explain how, under international law, actions by these various groups amount to armed attacks by Iran. In an apparent acknowledgement that these attacks by Iran’s supposed “proxies” cannot be legally attributed to Iran under principles of State responsibility, the letter frames Iran’s relationship with these groups as “[g]aming the international legal system.”
No Imminent Threat
The letter refers to the “imminence” of the threat posed by the Iranian regime. But the Trump administration has not demonstrated that Iran’s nuclear program, ballistic missiles, or other capabilities posed an imminent threat to either the United States or Israel. U.S. lawmakers briefed on the intelligence underlying the U.S. attack have asserted that there was no evidence that Iran posed an imminent threat. (Indeed, shortly before the war, Secretary of State Marco Rubio said that Iran was “not enriching right now, but they’re trying to get to the point where they ultimately can.”) In addition, the Director of the U.S. National Counterterrorism Center within the Office of the Director of National Intelligence in his March 17 resignation letter stated that “Iran posed no imminent threat to our nation.”
The Article 51 letter, however, asserts that the “imminence” of this supposed threat must “account for [Iran’s] decades of malign foreign and domestic conduct.” Thus, the Article 51 letter cites a litany of Iranian bad acts dating back to 1979—the sacking of the U.S. Embassy in Tehran and seizure of U.S. diplomats as hostages as well as the Marine barracks and Khobar Towers bombings, roadside bomb attacks against U.S. forces in Iraq, and Hamas’s atrocities of October 7th.
As horrific and unlawful as many of these actions were, they do not individually or collectively give rise to a right for the United States or Israel to use force in self-defense against Iran in 2026. Whatever right of self-defense the United States may have had against Iran in relation to the 1979 hostage crisis, it long ago expired, including as a result of the Algiers Accords and the freeing of U.S. hostages. The United States has not had an ongoing, 47-year-old right to use force in self-defense against Iran. Notably, the United States did not argue that the U.S. actions in the 1987-88 Tanker War with Iran were somehow justified by events in 1979.
An Unnecessary Use of Force is an Unlawful Use of Force
The fundamental problem with any attempt to justify Operation Epic Fury on the basis of self-defense is that the U.S. attack was unnecessary. Far from being a war of necessity, the Iran War is a war of choice. And it was Trump’s choice.
Following what he viewed as the brilliant military spectacle of “Operation Absolute Resolve”—the U.S. decapitation raid into Venezuela in early January—Trump was emboldened to take dramatic military action elsewhere. He issued threats of military action against not just Iran, but also Mexico, Colombia, and Cuba, as well as renewing his threat to annex Greenland. Indeed, Trump reportedly came close to attacking Iran in the immediate aftermath of the Venezuela operation, having threatened to intervene if the Islamic Republic cracked down against anti-regime protesters, but delayed military action due to concerns from Israel and U.S. Central Command that defenses against Iranian retaliatory attacks were not adequately prepared.
The president’s own words suggest the U.S. attack on Iran was not a result of any objective threat that Iran posed but instead of the U.S. president feeling invincible after the special operations forces raid into Caracas and seeking another dramatic military spectacle. Indeed,the president has repeatedly invoked Venezuela in connection with Iran and referred to the U.S. war on Iran as a “performance.” (“I hope you are impressed. How do you like the performance? I mean, Venezuela is obvious. This might be even better. How do you like the performance?”) This letter fails to offer a persuasive account to the contrary—that somehow U.S. military action was indeed necessary to respond to some threat.
The U.S. letter’s assertion that “peaceful measures have once again been attempted and exhausted” and therefore U.S. military action was necessary also ring hollow. It was of course the first Trump administration that upended the diplomatic resolution to constrain Iran’s nuclear program, in 2018 withdrawing the United States from the multilateral nuclear deal with Iran that the Obama administration had negotiated and implemented along with the other permanent members of the Security Council and European allies. The most recent U.S. nuclear diplomacy with Iran—led by U.S. special envoy Steve Witkoff and the president’s son-in-law Jared Kushner—did not involve career diplomats or technical experts to anywhere near the extent as the Obama-era nuclear deal. Rather than negotiations having reached a dead end, according to the Omani mediators speaking shortly before the U.S. attack on Iran, negotiations were due to continue the following week with technical discussions. Without involving the U.S. government’s own subject matter experts and professional diplomats, the administration cannot credibly claim to have exhausted negotiations. Rather than the exhaustion of diplomacy rendering the use of force necessary, the specific timing of the U.S.-Israeli attack on Iran was reportedly driven by an opportunity to launch a decapitation strike against Iran’s Supreme Leader.
The Use of Force is Illegal for Good Reasons
As the International Military Tribunal at Nuremberg announced in its 1946 judgment:
To initiate a war of aggression, therefore, is not only an international crime; it is the supreme international crime differing only from other war crimes in that it contains within itself the accumulated evil of the whole.
With the evils of two world wars and the Holocaust in mind, the United States played a critical role in enshrining the prohibition on the use of force in Article 2(4) of the U.N. Charter.
Unfortunately, the Trump administration’s war of choice in the Middle East has already reminded us of some of the reasons for banning the use of force. Over a thousand people have already been killed—including around 170 people (most of them children) in a school attacked by U.S. missiles as well as over a dozen U.S. servicemembers. Millions of people have been displaced from Iran to Lebanon in this now regional conflict. The price of oil has spiked and global markets have been rocked.
And notwithstanding the fact that the United States fulfilled the procedural requirements of the U.N. Charter by submitting this letter, leading scholars describe the U.S. attack on Iran as not only a clear violation of Article 2(4) of that treaty but a war of aggression. Indeed, close allies like the United Kingdom (at least initially) and Spain restricted military cooperation with the United States in order to avoid complicity in what they deemed an illegal war. Recent statements by an array of European leaders making clear they will reject joining the war effort (following Trump’s calls on them to aid militarily in reopening the Strait of Hormuz) reinforce this conclusion.
In terms of its broader ramifications, this war of choice will further damage the international legal order that the United States took a leading role in building, and from which the United States has long benefited. At the very least, the war on Iran creates additional negative precedent that States will see as eroding the prohibition on the use of force. The knock-on practical implications could include increasing the likelihood of nuclear proliferation as more States seek the ultimate atomic insurance policy—particularly as Iran’s nuclear threshold status proved an inadequate deterrent to military intervention.
It is the responsibility of senior executive branch lawyers and the president’s senior advisors to impress upon him the reasons why both the U.N. Charter and the U.S. Constitution sharply restrict recourse to military action. In an administration that seems to have low regard for the rule of law this may sound absurd, but there is nothing absurd about expecting senior officials, invested with the public trust, to do their jobs. Trying to paint an unnecessary war with a thin veneer of legality after the fact is not enough. Legal restrictions on the use of force—even if not framed in those terms—must be brought to bear early in the decision-making process, particularly when the consequences of those decisions are so high.
