President Donald Trump’s war against Iran is widely regarded as a serious breach of the U.N. Charter, in addition to the U.S. Constitution and the War Powers Resolution. As Tess Bridgeman and Oona Hathaway have described it, this is a triply illegal war. The State Department has tried to address some of those criticisms, but, as one of us has written, the official position is legally unpersuasive and analytically confused.
Regardless of those discussions, the U.S. military’s enforcement of the ongoing naval blockade of Iran may have now resulted in fresh violations of the U.N. Charter against four entirely different countries.
On May 29th, the U.S. military attacked a Gambia-flagged vessel in the Gulf of Oman. According to U.S. Central Command:
U.S. Central Command (CENTCOM) forces observed M/V Lian Star transiting international waters toward an Iranian port on the Gulf of Oman and issued more than 20 warnings while informing the vessel it was in violation of the U.S. blockade.
A U.S. aircraft disabled the vessel by firing a Hellfire missile into the ship’s engine room after Lian Star’s crew failed to comply. The ship is no longer transiting to Iran.
U.S. forces subsequently fired on at least four other third-country merchant vessels—a Botswana-flagged tanker, a Guinea-Bissau-flagged tanker, and two tankers flagged to Palau. The U.S. attack on one of the Palau-flagged tankers killed three Indian sailors. (Under the Compact of Free Association with Palau, the “United States has full authority and responsibility for security and defense matters in or relating to Palau.”)
In addition, in April, U.S. forces boarded another Botswana-flagged tanker in the Bay of Bengal, but claimed it was stateless. (The U.S. government has not to our knowledge asserted the other vessels were stateless.) As of June 11th, CENTCOM reports that since its implementation of the blockade on April 13, U.S. forces have disabled “nine non-compliant vessels” and redirected 135 ships.
These U.S. attacks on vessels flagged to States that are not a party to the conflict between the United States and Iran foreground a tension between two bodies of law:
- the law of naval blockade and the principles of neutrality law it incorporates; and
- the modern U.N. Charter regime governing the use of force between States.
As Professor Magne Frostad noted in 2024 with respect to blockades, a “routine use of the rules of naval warfare would then at the same time be a grave violation of the UN Charter.”
This tension has historically been largely theoretical, as interstate warfighting has taken place predominantly on land and in air since the U.N. Charter took effect (with notable exceptions such as the Falklands War and the Tanker War, discussed below). As a result, States have not fully reconciled the rules governing blockades, or naval operations more broadly, with the U.N. Charter regime. The void in State treatymaking has led to expert-led endeavors to craft manuals on the current law of naval operations. States, too, have issued updated manuals on naval warfare that have included language and even entire sections on the Charter. But those manual-drafting processes, given their mission to explain the law as it is rather than to move the needle, are stymied by the lack of State lawmaking on naval warfare in the modern era. The result is typically a disclaimer upfront that operations must be undertaken in accordance with the U.N. Charter, or even a statement, such as that in the German Commander’s Handbook, that the Charter obligations “will take absolute precedence over the ones arising from the law of maritime neutrality.” The operative sections that lay out the practical rules, however, often proceed many pages or even chapters after such disclaimers, and include rules that would permit acts inconsistent with the Charter. Given that jus ad bellum decisions and lawyering tend to happen at the strategic rather than the operational level of decision-making, as noted in the Newport Manual, the resulting guidance is insufficient (or simply misleading) for the people relying on these rules, leaving operators with insufficient information on how the Charter and other postwar legal developments should interact with the law of naval operations. And in cases like those addressed here, the manuals may seem to justify operations that are incompatible with the Charter.
Finally, although the legal issues here do not touch on Iran’s own military threats and attacks against maritime traffic in the Strait of Hormuz (which are widely viewed as illegal), we note that others have identified those abuses well.
Neutrality and Blockade
Under treaty and customary international law prior to the 20th Century—when war and the use of force were permissible tools of statecraft—the law of neutrality regulated the relationship between States not involved in an ongoing war (neutrals) and the warring States (belligerents). Neutral States were generally obligated to comply with requirements of (a) non-participation in the conflict and (b) impartiality as between the belligerent parties. Neutrality law also governed the responses by belligerents when a neutral State violated its obligations of neutrality. In the case of serious and systematic violations of neutral duties, such retaliation might include a declaration of war against the neutral, changing that State’s status should a conflict ensue. States fighting alongside one another were considered to be “co-belligerents.” But neutrality, or its violation, was never the legal basis for using force. Rather neutrality functioned as a legal shield, protection that a State could invoke in an era when the use of force was otherwise generally permissible. In the post-Charter world—as one of us has written in a recent article–that shield has become obsolete because inviolability is now the default: Article 2(4) extends its protection to all States, not merely neutral ones, and its exceptions are much narrower than those of neutrality law. That neutrality played the role of shield rather than sword is a critical distinction to understanding neutrality law’s occasional misuse today. Treating neutrality violations as an independent basis for using force today inverts the original logic—importing a limitation on protection from one legal regime into a different regime where a broader default protection now governs.
One domain of international law where the language of neutrality law persists is the law of naval warfare, specifically with respect to naval blockades.
The law of naval blockades applies only during armed conflict. The purpose of a naval blockade is to prevent the adversary from receiving supplies, as well as to prevent the export and sale of products that would sustain the adversary’s military capabilities. For a blockade to be lawful, it must: 1) have been publicly declared; 2) apply impartially to vessels of all nations (including neutrals); and 3) be effective, that is sufficient military assets must be deployed to implement it (e.g.,. not a paper blockade). (See here, here, here, and here for recent discussions of blockades with respect to U.S. actions against Venezuela, Cuba, and Iran.)
Historically, under the law of naval warfare, the blockading State might exercise “belligerent rights” to enforce it, including the “right of visit and search.” Merchant vessels believed on reasonable grounds to be breaching a blockade could be captured. Any vessel that was reasonably believed to be breaching the blockade could be captured and condemned in prize court, with the property title passing to the blockading State.
Further, under the law of naval warfare, merchant vessels which, after prior warning, clearly resist capture could be attacked. According to a leading expert manual on the law of naval warfare (the Newport Manual), neutral merchant vessels become liable to attack under that body of law by “[a]ctively resisting visit, search, or capture” as well as “refusing an order to stop.”
Critically, the law of naval warfare, including the rules regarding blockade, developed and were codified prior to the conclusion of the U.N. Charter. Yet all of these rules remain on the books in modern manuals describing the law of naval operations, as well as in expert-led treatises.
In the context of the current U.S.-Iran War, the U.S. military announced in a notice to mariners a naval blockade of Iran on April 13—notably after the ceasefire had already begun.
This blockade will be enforced in the Gulf of Oman and Arabian Sea east of the Strait of Hormuz. The blockade encompasses the entirety of the Iranian coastline to include but not limited to ports and oil terminals. The blockade applies to all vessel traffic, regardless of flag. Any vessel entering or departing the blockaded area without authorization is subject to interception, diversion, and capture. The blockade will not impede neutral transit passage through the Strait of Hormuz to or from non-Iranian destinations. Neutral vessels may still be subject to the right of visit and search to determine the presence of contraband cargo. Humanitarian shipments including food, medical supplies, and other goods essential for survival of the civilian populations will be permitted, subject to inspection.
As Mark Nevitt has noted, CENTCOM subsequently announced on April 16, an expansion of maritime enforcement operations beyond the scope of the blockade itself, including a belligerent right to board vessels subject to US sanctions—regardless of location.
U.N. Charter
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.” The provisions of the Charter—including Article 2(4)—take priority over other international legal obligations states may have. Article 103 provides that:
In the event of a conflict between the obligations of the Members of the United Nations under the present Charter and their obligations under any other international agreement, their obligations under the present Charter shall prevail.
The prohibition on the use of force codified in Article 2(4) is subject to two narrow exceptions specified in the text of the treaty. Article 51 of the Charter, 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 force in individual or “collective self-defense” (coming to the aid of another country exercising its right of self-defense) is permissible and must be reported to the U.N. Security Council.
The U.N. Security Council may also authorize the use of forcible measures by member States. Article 42 of the Charter specifically cites blockades as one such forcible measure:
Should the Security Council consider that measures provided for in Article 41 would be inadequate or have proved to be inadequate, it may take such action by air, sea, or land forces as may be necessary to maintain or restore international peace and security. Such action may include demonstrations, blockade, and other operations by air, sea, or land forces of Members of the United Nations.
The U.N. Security Council may also authorize the use of forcible measures by member States. Article 42 of the Charter specifically cites blockades as one such forcible measure:
Should the Security Council consider that measures provided for in Article 41 would be inadequate or have proved to be inadequate, it may take such action by air, sea, or land forces as may be necessary to maintain or restore international peace and security. Such action may include demonstrations, blockade, and other operations by air, sea, or land forces of Members of the United Nations.
As Michael Schmitt and Rob McLaughlin previously explained in Just Security, “‘blockades’ qualify as a use of force even before force is used against ships attempting to breach them.” The Trump administration also recently acknowledged that “a military blockade constitutes a use of force for the purposes of UN Charter Article 2(4).” (Aside: The continued enforcement of the blockade is thus inconsistent with the Trump administration’s claims of a simultaneous ceasefire and that hostilities terminated at the time the ceasefire began.)
Given that the U.S. war on Iran is unlawful as a matter of jus ad bellum—in other words, in breach of the U.N. Charter—the blockade on Iran as part of that ongoing use of force is also unlawful. It is unlawful as a matter of jus ad bellum whether or not the blockade is being conducted in accordance with the laws of armed conflict, also called the jus in bello. These are two distinct boxes that must be checked for the State’s actions to be lawful. Jus ad bellum governs the State’s right to resort to force, and jus in bello governs the conduct of hostilities inside that armed conflict. So the U.S. blockade and acts to enforce it may well satisfy the latter, but not the former.
In this case, there are additional uses of force against third States that must be addressed.
Attacks on Third States’ Merchant Vessels and the U.N. Charter
Given their nature, blockades at times involve the threat or use of force against States that are not parties to the primary conflict. The blockade itself constitutes a use of force against the blockaded State. But a blockade may also involve incidents between the blockading State and vessels flagged to third party States. Such incidents, such as those reported in recent days, may involve an actual attack by the blockading State on such vessels, which could be construed as a use of force against the vessel’s flag State. To put a finer point on it, the United States has taken the position (discussed below), in other contexts, that attacks upon flagged merchant vessels constitute a use of force (which in the U.S. view is equivalent to an armed attack) against the flag State. Under the Charter, armed attacks give rise to a right of self-defense under Article 51 of the Charter.
Other States have also historically taken the position that attacks on flagged vessels amount to a use of force, with some further calling it “aggression,” which suggests that they viewed them as amounting to an armed attack. In one noteworthy early example, during border tensions between Guatemala and Mexico in 1958-59, the Mexican President invoked Article 51 of the Charter in response to Guatemala’s attack upon Mexican shrimping vessels (though Mexico did not actually use force in response). According to contemporary reporting by the New York Times, “Mexican authorities … maintained that in any case the strafing of unarmed boats of a friendly country amounted to an act of aggression.”
During the Iran-Iraq War, both belligerents attacked merchant vessels—including oil tankers— in the Persian Gulf and Strait of Hormuz. (Neither Iran nor Iraq seem to have declared nor sought to enforce a traditional blockade.) During meetings of the U.N. Security in 1984, representatives from multiple countries indicated that such attacks violated the Charter. Tunisia stated that attacks on “unarmed merchant vessels” (including oil tankers) “run counter to the obligations assumed by their perpetrators under the Charter of the United Nations” and “are also a violation of the sovereignty of the countries concerned.” Djibouti denounced the “aggression of the Iranian war machine against oil-tankers and other commercial vessels.” Qatar also labelled Iranian attacks on Kuwaiti and Saudi tankers as “aggression.”
For its part, the United States told the Security Council that:
We therefore agree with the States members of the Gulf Co-operation Council that the Security Council should take a clear and unambiguous stand against the extremely dangerous expansion of the war by attacks on innocent vessels in international waters or in the territorial waters of non-combatants. We firmly support the views expressed by the Gulf Co-operation Council and the League of Arab States on this issue.
The United States subsequently responded to continued Iranian attacks on neutral oil tankers by reflagging Kuwaiti vessels as American and accompanying them with U.S. naval convoys as part of Operation Earnest Will. The Legal Adviser to the State Department argued at the time, “U.S. protection of the vessels is intended to deter rather than provoke military action by Iran.” As it turned out, this operation led to repeated hostilities between U.S. and Iranian forces in 1987-1988—including as a result of an Iranian attack on a reflagged Kuwaiti vessel.
As recounted in the U.S. letter to the UN Security Council, “on 16 October 1987, a Silkworm missile fired by Iranian forces from Iranian-occupied Iraqi territory struck the Sea Isle City, a United States flag vessel, in the territorial waters of Kuwait.” In response, the United States destroyed an Iranian oil platform, putatively in self-defence, an act it could not lawfully take unless it considered the Iranian action a violation of the Charter and an armed attack.
President Reagan himself weighed in on the Iranian attack and the U.S. response:
Iran’s unprovoked attacks upon U.S. and other nonbelligerent shipping, and particularly deliberate laying of mines and firing of Silkworm missiles, which have hit U.S.-flag vessels …. The action against the Iranian military platform came after consultations with congressional leadership and friendly governments. It is a prudent yet restrained response to this unlawful use of force against the United States and to numerous violations of the rights of other nonbelligerents. It is a lawful exercise of the right of self-defense enshrined in article 51 of the United Nations Charter and is being so notified to the President of the United Nations Security Council. (emphasis added)
In the subsequent Oil Platforms case before the International Court of Justice, the United States took the position that:
Sea Isle City was properly registered in the United States and flew the U.S. flag in compliance with all applicable standards of international and U.S. law. She was a U.S.- flag vessel, and the attack upon her justified acts of self-defense by the United States. (emphasis added).
In arguing that the United States could invoke the right of self-defense in response to an attack upon this formerly Kuwaiti vessel, the United States explained that:
Under international law, States have the right to confer their nationality on ships by registering the ship, authorising it to fly its flag, and issuing papers documenting the ship’s nationality. There are two fundamental limitations on registration of vessels. First, vessels may only be registered to one state. Registration of Sea Isle City was transferred exclusively to the United States.
Second, there must be a “genuine link” between the ship and its flag State.
The United States contended that the “domestic measures taken by the United States in connection with Sea Isle City fully met these requirements” for a “genuine link.”
The ICJ did not squarely address the U.S. position that the attack on a flagged vessel was an attack upon the flag State. The court raised questions of attribution and intent regarding the attack on the Sea Isle City and indicated that it did not view that and related incidents singularly or cumulatively as constituting an “armed attack” on the United States.
In the context of the Houthis’ post-October 7th attacks on commercial shipping in the Red Sea and subsequent U.S./U.K. military responses, the United States and other States also cited attacks on merchant vessels. The United States repeatedly invoked imminent threats to both “U.S. Navy ships and merchant vessels” as the predicate for U.S. “self-defense” strikes in Yemen. In its January 12, 2024 Article 51 letter to the United Nations regarding these strikes, the United States asserted that U.S.
strikes were conducted to degrade and disrupt the ongoing pattern of attacks threatening the United States and deter the Houthi militants from conducting further attacks threatening merchant and commercial vessels transiting the Red Sea…The United States took this necessary and proportionate action consistent with international law and in the exercise of the United States’ inherent right of self defence as reflected in Article 51 of the Charter of the United Nations.
The same day, in a joint statement issued the same day Australia, Bahrain, Canada, Denmark, Germany, Netherlands, New Zealand, Republic of Korea, United Kingdom, and the United States, explained:
In response to continued illegal, dangerous, and destabilising Houthi attacks against vessels, including commercial shipping, transiting the Red Sea, the armed forces of the United States and United Kingdom, with support from the Netherlands, Canada, Bahrain, and Australia, conducted joint strikes in accordance with the inherent right of individual and collective self-defence, consistent with the UN Charter, against a number of targets in Houthi-controlled areas of Yemen.
And the U.N. Security Council in Resolution 2722 in response to Houthi maritime attacks, affirmed:
the exercise of navigational rights and freedoms by merchant and commercial vessels, in accordance with international law, must be respected, and takes note of the right of Member States, in accordance with international law, to defend their vessels from attacks, including those that undermine navigational rights and freedoms …
Of particular significance in this regard, the U.S. Navy takes the position that attacks on flagged vessels implicate the flag State’s right of self defense. The 2022 edition of the Commander’s Handbook on the Law of Naval Operations (3.10.1), provides that
International law, embodied in the doctrines of self-defense and protection of nationals, provides authority for the use of proportionate force by U.S. warships and military aircraft when necessary for the protection of U.S.-flagged vessels …. ROE are carefully constructed to ensure the protection of U.S.-flagged vessels.
The U.S. position regarding attacks on flagged vessels is not unquestioned. Some scholars point to the definition of acts of aggression in Article 3(d) of the UN General Assembly Resolution 3314, which speaks of the “an attack by the armed forces of a State on the … marine or air fleets of another State.” The counter-argument runs that a single merchant ship is not a “fleet” and thus an attack on an individual merchant vessel would not constitute an act of aggression. Further complicating this debate are the distinctions between the use of force, armed attacks, and acts of aggression under international law. The United States has historically regarded any use of force as an armed attack.
Whether or not the proposition is universally accepted, it has been the U.S. position that an attack upon a flagged vessel is an armed attack giving rise to the right of self-defense by the flag State.
Article 2(4) vs Belligerent Rights
The factual parallels between the Iranian missile attack on the Sea Isle City in 1987 and the U.S. missile attacks on the Lian Star in 2026 highlight the tension between the interpretation of the U.N. Charter with respect to forcible actions against merchant vessels and the exercise of belligerent rights against such vessels during the enforcement of a blockade. Unlike self-defense or U.N. Security Council authorization, the exercise of belligerent rights to enforce a blockade is not a recognized exception to Article 2(4)’s prohibition on the use of force.
Thus, outside of a blockade authorized by the Security Council under Article 42, there does not appear to be any jus ad bellum basis for the use of force against neutral merchant vessels in the enforcement of a blockade. Indeed, under the United States’ own view, the use of forcible measures against a merchant vessel may amount to an armed attack upon the flag State, triggering a right of self-defense by that State. Further, even the threat of forcible measures against neutral merchant vessels would (consistent with the U.S. position taken at the ICJ) constitute an unlawful threat of the use of force, in violation of Article 2(4).
The U.S. Commander’s Handbook on the Law of Naval Operations acknowledges Articles 2(4) generally (5.1.1) and specifically that the “customary law of neutrality has, to some extent, been modified by the Charter of the UN.” (7.2.2) The Handbook does not explicitly address however how the Charter might bear on the enforcement of blockades.
The lack of clarification on how to interpret the law of naval warfare consistently with the Charter is now coming to a head. The exercise of belligerent rights through attacks on vessels flagged to non-party States and, by extension, a blockade involving the use of force against merchant vessels that are not party to the armed conflict is in clear and direct tension with the U.N. Charter as interpreted by the United States and many others. And this is irrespective of whether there is a lawful basis to use force against the blockaded State itself. If a State has a jus ad bellum basis to use force against another State to repel an armed attack, it may use the force that is necessary and proportionate to do so. This does not extend to strikes on third States that have not themselves provided cause for the use of force in self-defense.
Conclusion
President Trump’s war on Iran has been characterized by what many legal experts consider to be serious violations of not only the international and U.S. domestic rules governing the resort to force, but also possibly the law of armed conflict in waging hostilities.
The nature of some of these violations is unique to the Trump administration. For example, it is difficult to imagine a previous Secretary of Defense declaring “no quarter”—an announcement which could itself constitute a war crime. Similarly, no other U.S. president has issued threats to destroy all of a country’s power plants or stated that a “whole civilization will die tonight,” utterances which themselves could violate the law of war as “threats of violence the primary purpose of which is to spread terror among the civilian population.”
But other possible transgressions by the Trump administration are surfacing longstanding problems in law and legal practice. The threat and use of force against neutral merchant vessels in the enforcement of the blockade of Iran foreground a long-running, though previously largely theoretical, tension between two distinct legal regimes. In one legal regime—which developed before the U.N. Charter—the use of force was a permissible tool of statecraft. In the other legal regime, codified in the U.N. Charter, the use of force is generally prohibited. The U.S. attacks on commercial vessels flagged to third States and threats of the use of force against other commercial vessels to enforce the blockade of Iran may represent a point at which the two regimes are irreconcilable–at least under the United States’ own past interpretation of the U.N. Charter.
The resurgence of naval warfare suggests it is urgent to address these tensions. This is not the only area where the prohibition on the use of force has come under stress. But efforts to fortify the legal system need to address these chronic conflicts in the law of naval operations broadly, and contradictions of the United States’ approach to military action more specifically. One necessary reform will be acknowledging that the U.N. Charter supersedes pre-Charter legal regimes, including with respect to neutrality and naval warfare.
Thanks to Mark Nevitt and Mike Schmitt for their thoughtful comments on a draft of this piece. Special thanks as well to Marie Miller for excellent and speedy research assistance with this piece.
