Targeting Merchant Ships in the Strait of Hormuz: The Iran–U.S. MoU and the Resumption of Hostilities

[Amir Abbas Kiani is a collaborating researcher in International Law at Shiraz University, Iran.]

On June 17, 2026, the Islamic Republic of Iran and the United States of America (U.S.) signed an agreement, formally entitled the ‘Islamabad Memorandum of Understanding’ (MoU), to resolve certain disputes arising from the international armed conflict (IAC) that broke out between them on February 28, 2026, address other disputes beyond the scope of that conflict, and, in particular, suspend hostilities (ceasefire). Nevertheless, since the signing of the MoU, the exchange of fire has not effectively ceased, and the parties have repeatedly engaged in hostilities against one another despite paragraph 1 of the document, which provides that both parties “…declare upon the signing of this [MoU] an immediate and permanent end to the war on all fronts…and undertake that from now on they will not launch any hostile action against each other, and will refrain from the threat or use of force against each other.”

One of the principal points of dispute concerning the MoU pertains to paragraph 5, which addresses the arrangements governing the Strait of Hormuz. Paragraph 5 provides: “Upon signing this [MoU], the Islamic Republic of Iran will immediately take steps to ensure that the movement of merchant ships from the Persian Gulf to the Sea of Oman and vice versa is resumed within 30 days to the pre-war volume, taking into account the need for the removal of technical obstacles and the neutralization of mines by Iran.”

However, hostilities resumed when Iran “fired on three commercial vessels… in Oman’s territorial waters near the Strait of Hormuz, according to a U.S. official. The official described the strikes as a ‘gross violation’ of the memorandum of understanding with Iran.” Following these attacks, the U.S. resumed its military campaign against Iran. In a statement, the U.S. Central Command (CENTCOM) reported that it “completed a new round of offensive strikes against Iran, July 7, hitting over 80 targets with precision munitions as an immediate response to Iran’s latest attacks on commercial vessels transiting the Strait of Hormuz.” Notably, the U.S. justified its renewed military operations on the ground that Iran had failed to comply with its obligations under paragraph 5 of the MoU concerning “the movement of merchant ships from the Persian Gulf to the Sea of Oman and vice versa” through the Strait of Hormuz.

This situation raises some fundamental legal questions: Are the U.S. and Iran entitled to resume hostilities in response to an alleged breach of the obligations contained in the MoU, particularly paragraph 5? More fundamentally, is it the MoU itself, or rather the underlying IAC between the two States, that constitutes the relevant legal basis for assessing the lawfulness of any subsequent resumption of hostilities, such that a ‘material breach’ of the MoU would simply entitle the injured party to resume hostilities? Or must that question continue to be governed by the rules on the use of force (jus ad bellum)?

The MoU as a Ceasefire Agreement

In summary, a ceasefire is “an agreement that effectuates the suspension of ongoing hostilities between opposing belligerents.” However, it is well-established that ceasefire agreements do not end the armed conflict until the ‘general close of military operations’ test, as enshrined in Article 3(b) of Additional Protocol I to the 1949 Geneva Conventions, is satisfied. As provided by the International Criminal Tribunal for the former Yugoslavia Trial Chamber I of the Gotovina case, the legal test is whether “the [IAC] had found a sufficiently general, definitive and effective termination so as to end the applicability of the law of armed conflict” (para. 1694) (emphasis added) (for further analysis on this issue, see here).

Many instances of the suspension of hostilities are literally referred to as ‘ceasefires.’ Even so, different terms have been used to describe this situation with distinct yet interrelated legal effects: ‘ceasefire’ and ‘armistice.’ In short, as has been observed by Christine Bell, “ceasefire indicates a temporary or permanent cessation of hostilities with or without normalization of relations, (para. 6)… armistice connotes a permanent cessation of hostilities without normalization of relations” (para. 5) (emphasis added). The U.S. Department of Defense Law of War Manual considers armistice as equal to  truce, local truce, cease-fire, cessation of hostilities, and suspension of arms (§ 12.11.1.3).

Viewed in this light, the MoU can be best characterized as a ceasefire agreement intended to suspend, rather than terminate, active military operations. Until the ‘general close of military operations’ test is satisfied, the underlying IAC continues notwithstanding the parties’ agreement to suspend hostilities. Yet, this conclusion does not, by itself, determine the lawfulness of any subsequent resort to force, which remains a distinct legal question.

The Strait of Hormuz: Interpreting Paragraph 5 of the MoU       

Pursuant to paragraph 5 of the MoU, Iran is under an immediate obligation to “…take steps to ensure that the movement of merchant ships from the Persian Gulf to the Sea of Oman and vice versa is resumed within 30 days to the pre-war volume…”. Nonetheless, Iran interprets this paragraph as meaning that merchant ships must navigate using the routes it has declared.

It must be noted that nothing in the text signifies that the merchant ships must navigate along the routes designated by Iran. At the same time, prior to the conclusion of the Islamabad MoU, Iran had consistently asserted that commercial vessels transiting the Strait of Hormuz were required to comply with Iranian routing and security requirements. Nothing in Iran’s subsequent practice indicates that it abandoned this position after the MoU. Rather, the parties’ subsequent dispute over paragraph 5 suggests that Iran continued to interpret the provision as permitting navigation in line with routes designated by the Iranian authorities, whereas the U.S. interpreted it as guaranteeing unrestricted commercial passage without such routing requirements.

The dispute over paragraph 5 must be resolved under Article 31 of the Vienna Convention on the Law of Treaties (VCLT), which requires treaties to be interpreted in good faith according to their ordinary meaning, context, and object and purpose. Two contrasting approaches have shaped treaty interpretation: Sir Humphrey Waldock emphasized a contextual and evaluative assessment of the text, circumstances, and purpose, while Sir Hersch Lauterpacht placed greater weight on the parties’ intentions and preparatory works (travaux préparatoires) (Yearbook of International Law Commission (ILC), 1964, Vol. II, pp. 53-54). In general, treaty interpretation should primarily focus on the treaty’s text, which reflects the parties’ agreed bargain, rather than broad assumptions about its purpose or the drafters’ intentions. Accordingly, gaps or silence should not be treated as permission to adopt interpretations unsupported by the text (p. 61). The International Court of Justice (ICJ) affirmed this approach in Maritime Delimitation and Territorial Questions (Qatar v. Bahrain), holding that the parties’ actual commitments, as expressed in the signed instrument, prevail over any subsequent claims about their subjective intentions (paras. 27, 29).

Therefore, Iran’s interpretation of paragraph 5 is difficult to sustain: the text says nothing about specific passage routes, and this reading is better understood as belonging to the future ‘final agreement’ rather than the MoU itself. Moreover, from an effet utile perspective, Iran’s reading risks depriving paragraph 5 of any independent effect, since it amounts to little more than a restatement of the pre-existing routing practice that Iran maintained regardless of the MoU, precisely the practice that gave rise to resumption of hostilities in the first place.

On the other side, Iran may posit that, since it is the party bound by this obligation, it is entitled to determine how to comply with its commitment. With respect to this measure of ‘discretion’, the question therefore arises as to the nature of paragraph 5, particularly in light of the traditional distinction between ‘obligations of conduct’ and ‘obligations of result’. Iran may argue that its obligation under paragraph 5 is an obligation of result, the required result being “the movement of merchant ships…to the pre-war volume”. This reasoning can be reinforced by the ILC’s first reading of the Draft Articles on State Responsibility, where it expressed the view that “…the freedom of choice accorded to the State is implicit in the fact that the international obligation only specifies the result to be achieved, the text imposing the obligation making no reference at all to the means of achieving it…” (p. 141). Hence, Iran might hold that, because it alone bears the obligation to remove technical obstacles and neutralize mines, it retains discretion as to how that removal is carried out, including through a designated traffic-management scheme, while the revival of the ‘pre-war volume’ of movement throughout the Strait of Hormuz constitutes the resulting purpose. Critics, however, may put forward that this reasoning is potentially self-serving: the same route designations that Iran invokes as an exercise of its discretion are also the source of the practical friction in the Strait of Hormuz.

Does an Alleged Breach of the MoU Provide a Legal Basis for the Resumption of Hostilities?

One approach holds that since the ceasefire agreement does not terminate an armed conflict and the law of armed conflict (LOAC) therefore remains applicable, the effect of a ceasefire on the parties’ subsequent entitlement to resume hostilities against each other should be considered carefully. In this regard, Article 60 of the VCLT states that a ‘material breach’ of a treaty entitles the injured party to invoke that breach as a ground for suspending or terminating the treaty. In short, a material breach involves the violation of a treaty provision that is fundamental to the treaty, with its essential character determined in light of the treaty’s object and purpose (pp. 574-576). As for the MoU, looking at paragraph 13 that regards implementation of paragraph 5 as “receipt of assurances…[to] enter negotiations for a Final Agreement…”, it would be reasonable to argue that paragraph 5 is an essential part of the MoU, the violation of which could amount to a ‘material breach’ in accordance with Article 60 of the VCLT.

On this view, Iran’s attacks on commercial vessels, themselves arguably inconsistent with its obligation under paragraph 5 of the MoU, may be seen as a material breach, entitling the U.S. to suspend or terminate its own obligations under the MoU. However, justifying the resumption of hostilities under Article 60 of the VCLT remains both unclear and highly controversial. This provision addresses the treaty-law consequences of a material breach, namely, the possibility of suspending or terminating the treaty, without itself establishing a right to recommence hostilities.  In this context, the question of resumption should be addressed by invoking Article 40 of the Hague Regulations (HR) which provides that “[a]ny serious violation of the armistice by one of the parties gives the other party the right to denounce it, and even, in case of urgency, to recommence hostilities at once” (emphasis added). In fact, Article 60 of the VCLT can be used to establish the existence of a ‘material breach’; however, for the purpose of assessing the plausibility of resumption of hostilities, it must be read together with Article 40 of the HR.

The Continued Relevance of Jus ad Bellum

More fundamentally, it is difficult to accept the view that jus ad bellum becomes irrelevant during a ceasefire simply because the IAC and the applicability of international humanitarian law (jus in bello) continue. (see e.g. here, pp. 54-56, here, pp. 129 and here, pp. 67-68). The narrow view holds that once force has been lawfully initiated and an armed conflict begins, the legality of subsequent military operations must be assessed primarily under jus in bello (p. 4), whereas the opposing view maintains that jus ad bellum and jus in bello continue to apply concurrently throughout the conflict. The latter approach thus rejects the idea that the commencement of an armed conflict displaces the continued relevance of jus ad bellum (p. 674).

This approach has been confirmed by the ICJ in its Advisory Opinion concerning the Legal Consequences arising from the Policies and Practices of Israel in the Occupied Palestinian Territory, including East Jerusalem in 2024. The ICJ based its determination of the illegitimacy of that presence on its evaluation of Israel’s transgressions of the principle prohibiting annexation, which became evident over time, in violation of Article 2(4) of the United Nations (UN) Charter (paras. 157-179) (see also declaration of Judge Charlesworth, para. 27). So, it appears that the ICJ supports the so-called ‘concurrent application’ of jus ad bellum and jus in bello throughout the entire duration of an IAC. Otherwise, a prolonged IAC could become trapped in a ‘never-ending’ legal framework in which the continued application of jus ad bellum is effectively displaced solely because the armed conflict has not formally come to an end. Thus, the scope of any resumed force remains subject to independent scrutiny under necessity and proportionality, which forms a substantial part of the jus ad bellum inquiry.

Nevertheless, it might be objected that this debate matters most for conflicts that have persisted and remained largely static, for years or decades, such as the Israeli-Palestinian conflict. The Iran-U.S. armed conflict, it might be said, is not of that character: it remains comparatively hot, and Iran’s own material breach of the MoU already furnishes the U.S. with a distinct legal basis, potentially under Article 60 of the VCLT and Article 40 of the HR, for resuming hostilities, arguably rendering the jus ad bellum inquiry unnecessary. Yet this objection conflates two distinct questions. Article 40 of the HR and Article 60 of the VCLT may determine whether the U.S. was released from its ceasefire obligations and entitled to recommence hostilities; they do not, however, necessarily determine whether the particular force employed in doing so was lawful under jus ad bellum. Nor does this mean that every recommencement of hostilities following a serious breach must be preceded by a wholly new ‘armed attack’. Such a requirement would risk depriving Article 40 of the HR of any effet utile and collapsing the distinction between a ceasefire and a peace treaty.

Accordingly, it would be incorrect to suggest that the MoU itself provides the legal framework for determining whether either party may lawfully resort to force. Nothing in the MoU’s own text expressly regulates the circumstances in which either party may lawfully resume the use of force, that question, as discussed above, falls to Article 40 of the HR and the general rules of jus ad bellum. On the contrary, paragraph 1 of the MoU provides that “they will not launch any hostile action against each other, and will refrain from the threat or use of force against each other.” In particular, while Iran’s material breach may have released the U.S. from its ceasefire obligations and entitled it, in a case of urgency, to recommence hostilities, the extent to which, and manner in which, the legality and scope of the resulting use of force should be assessed under the applicable rules of jus ad bellum remain less clear.

Concluding Remarks

The analysis above suggests that the legal consequences of a breach of a ceasefire agreement cannot be determined solely by reference to the instrument establishing the ceasefire. Where hostilities are suspended rather than definitively terminated, the legal framework governing their possible resumption remains more complex than the mere invocation of a breach. The Islamabad MoU illustrates the need to distinguish between the legal consequences of violating a ceasefire agreement and the separate question of the lawfulness of force subsequently employed. Ultimately, while a material breach may provide a basis for the resumption of hostilities, whether, and to what extent, the applicable rules of jus ad bellum continue to govern the force subsequently employed remains controversial in the particular circumstances of the Iran-U.S. conflict.

Photo attribution: Abolhassan Neghabi on Unsplash