Five “Blockades” and One Legal Problem: Naval Enforcement in the U.S.–Iran Conflict

There are not one, but five distinct blockade-style operations underway in the U.S.-Iran armed conflict. Each has different legal authorities, and each raises different legal issues and operational risks. The central problem is not simply that there are multiple blockade-like operations—it is that the United States is collapsing distinct legal authorities into a single “blockade” framework, obscuring the legal basis for each. These operations draw from distinct legal paradigms that are increasingly being blurred.

First, Iran blockaded the Strait of Hormuz on March 2—an illegal blockade that has been remarkably effective, bringing maritime traffic in the Strait to a standstill.

Second, U.S. Central Command (CENTCOM) responded on April 13 with a naval blockade applicable to all vessels transiting to or from Iran.

The United States began the third, fourth, and fifth blockades on April 16. CENTCOM dramatically expanded the initial blockade by asserting a belligerent right to visit and search (1) all Iranian vessels; (2) vessels suspected of carrying contraband; and (3) vessels with active Treasury Department Office of Foreign Assets Control (OFAC) sanctions—regardless of location. The Pentagon is characterizing all these enforcement actions as a wartime blockade – that’s problematic. This global enforcement expansion beyond the Strait raises a set of legal questions that should be clarified. Each visit, board, search, and seizure must rest on a sound legal foundation. These operations draw from three distinct legal frameworks: (1) the law of naval warfare; (2) peacetime maritime law enforcement authorities; and (3) hybrid regimes such as the Proliferation Security Initiative.

We may well be entering the most legally surreal moment of the conflict. The United States is simultaneously observing a ceasefire with Iran while enforcing a naval blockade—a belligerent wartime operation that has no legal basis in peacetime. Normally, the imposition of a naval blockade ends a ceasefire, because a blockade is itself a belligerent act. Not here, apparently – that is, in the view of the United States but not in the view of Iran (which considers the blockade a “blatant violation[]” of the ceasefire). The United States is neither fully at war nor fully at peace according to its own logic. While asserting wartime legal authorities to board neutral vessels on the high seas, the United States has both declined to exercise the full menu of wartime military operations (e.g., resumption of air strikes against Iran) and to specify the legal framework for each operation.

The coherence of the international legal regime in this space is vital to preserving rules that facilitate maritime trade and travel – matters of long-term interest to the United States and the world. Compliance with the existing law can also be important to other states in their determinations whether to support or resist U.S. efforts.

Category #1:  Iran’s Illegal Blockade and Mining of the Strait of Hormuz

When the United States and Israel attacked Iran in February, Iran responded by announcing an illegal blockade of the Strait of Hormuz on March 2. This unilateral suspension of transit passage through the Strait violates the prohibition on barring access to or departure from neutral ports and coasts. Since then, Iran has committed a trifecta of illegal actions: (1) an illegal blockade of the Strait; (2) illegally mining the Strait; and (3) firing upon neutral shipping attempting to break the blockade and exercise the lawful right of transit passage. As I have previously unpacked, Iran’s blockade, mining, and firing upon neutral shipping are all illegal under the law of the sea and laws of naval warfare.

Furthermore, Iran has reportedly lost track of the mines—which is not only an operational problem, but also an independent law of naval warfare violation under the Hague Convention VIII. And Iran is trying to “pull a Suez,” by attempting to toll a chokepoint as a source of revenue and coercion in exchange for safe passage through the Strait. Despite its illegality, the blockade of the Strait has proven to be remarkably effective as a lever of Iranian power while bringing commercial shipping through the Strait to a standstill.

Category #2: U.S. Blockade Against Vessels Transiting To or From Iranian Ports

On April 13, CENTCOM commenced enforcement of a naval blockade of all Iranian ports and coastal areas. The blockade announcement reads in full:

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.

The legal foundation for this second blockade—interdicting vessels transiting to or from Iranian ports—is more consistent with traditional blockade law. As former Navy JAG Todd Huntley has analyzed, the U.S. blockade satisfies the traditional legal requirements for a valid blockade under the law of naval warfare: it has been publicly declared, applies impartially to vessels of all nations, and (for now) appears to be effective due to sufficient naval power in the region (see also Michael Schmitt and Rob McLaughlin’s exposition of these conditions). The U.S. blockade of Iranian ports, as it was pronounced, permits humanitarian shipments and preserves neutral transit passage rights. The April 13 declaration limited the blockade enforcement to the Gulf of Oman and Arabian Sea east of the Strait of Hormuz—a clear geographical limitation that aids in meeting the effectiveness requirement of a blockade.

Of course, the blockade cannot and does not apply in neutral territory, such as the territorial seas of India and Pakistan. Ships entering or exiting the Strait of Hormuz may attempt to break the blockade by “hugging” India’s and Pakistan’s coastlines and invoking the right of innocent passage. But India and Pakistan are unlikely to permit such a creative maneuver as systematically allowing what amounts to blockade running is legally fraught and raises neutrality concerns.

The U.S. seizure of the M/V Touska on April 19 – which was reportedly en route to Bandar Abbas, Iran – falls in this second blockade category. The container ship was flying an Iranian flag and refused repeated orders to divert. Based on public reporting, a U.S. Navy destroyer lawfully used force against the Touska following verbal warnings, warning shots, and disabling fire into the engine room. The 31st Marine Expeditionary Unit forces conducted a non-compliant boarding, which was legally justified under the law of naval warfare.

April 16 Blockade Update: A Dramatic Expansion of Maritime Boarding Authorities

The legal foundation of the U.S. blockade began to erode three days later, when CENTCOM’s April 16 blockade update dramatically expanded enforcement well beyond those geographic and legal limits.  On April 16, CENTCOM updated the initial blockade to apply anywhere in the world, with the exception of “neutral territory.” The updated notice to the initial U.S. blockade states in full:

In addition to enforcing the blockade, all Iranian vessels, vessels with active Office of Foreign Assets Control (OFAC) sanctions, and vessels suspected of carrying contraband are subject to belligerent right to visit and search. These vessels, regardless of location, are subject to visit, board, search, and seizure.

The so-called blockade expansion effectively adds three additional maritime enforcement operations to bolster the underlying blockade. It asserts a worldwide, belligerent right to visit and board: (1) all Iranian vessels; (2) any vessel suspected of carrying contraband; and (3) any vessel with an active OFAC sanction imposed by the Department of the Treasury.  In particular, the April 16 notice’s assertion of a belligerent right to board OFAC-sanctioned vessels is legally problematic and overlays a wartime legal regime onto a law enforcement sanction.

Category #3: Global “Blockade” of All Iranian Vessels

Intercepting all Iranian vessels has a comparably strong legal basis in the law of naval warfare, but a global blockade without geographic limit lacks a solid legal basis to qualify as a blockade.  Rule 94 of the San Remo Manual states that blockade declarations must “specify the commencement, duration, location, and extent of the blockade,” and the U.S. Navy’s Commander’s Handbook states that a blockade declaration should include its geographic limits. Can a worldwide blockade qualify as a legal blockade?  Probably not, as a global blockade would undermine neutral rights and would be difficult, if not impossible, to enforce. On the latter, blockades have to be effective to be legal, and a global blockade is simply too large to credibly enforce.

However, legal authorities outside of blockade law do provide a separate legal basis. Iran is a belligerent in the armed conflict, and Iranian-flagged vessels are enemy vessels subject to capture under the law of naval warfare.  Under the San Remo Manual (Rule 135), enemy vessels may be captured on the high seas regardless of their cargo or destination—the flag alone is a sufficient predicate. And the U.S. Navy Handbook states that “enemy merchant vessels may be captured wherever located beyond neutral territory.”

The U.S. Navy needs only to establish that the vessel is Iranian-flagged to capture the vessel. Vessels engaged exclusively in humanitarian missions or entitled to special protection under international law retain those protections even in armed conflict. Small coastal fishing vessels and small boats engaged in local coastal trade are also exempt. Enforcement operations must comply with the principle of distinction and the rules governing the use of force during non-compliant boardings.

Category #4: Global “Blockade” of Vessels Carrying “Absolute” and “Conditional” Contraband 

Fourth, under the law of naval warfare, neutral vessels are liable to capture by belligerent warships if they are carrying contraband. This view is reinforced in § 7.10 of the U.S. Navy’s Commander’s Handbook on the Law of Naval Operations, § VI of the San Remo Manual on International Law Applicable to Armed Conflicts at Sea, and the influential Newport Manual on the Law of Naval Warfare. The San Remo Manual (Rule 148) is specific that absolute contraband may be captured outside neutral waters, not just in the theater of conflict.

The CENTCOM notice provides a non-exhaustive list of contraband items that are subject to capture at any place beyond neutral territory if the destination “is the territory belonging to or occupied by Iran.” Contraband is defined as “goods that are destined for an enemy, and that may be susceptible to use in armed conflict.” CENTCOM has identified “absolute contraband” to include a variety of weapons, ammunition, and equipment, while “conditional contraband” encompasses petroleum products and lubricants due to their “essential role in military operations.”  These vessels, regardless of location, were now subject to the belligerent right to visit and search by the U.S. military.

CENTCOM’s notice also defines petroleum, oil, and lubricants as conditional contraband “due to their essential role in military operations and their contribution to Iran’s war-sustaining economy,” which is remarkably broad. Historically, conditional contraband requires a showing of destination for military use, not merely contribution to a general war-sustaining economy.  As Mike Schmitt and others have noted, a critical factor in treating goods as contraband is the cargo’s destination if it is transited by neutral merchant vessels.  There needs to be intelligence showing that the petroleum is actually destined for the Iranian military’s use; oil exported for commercial purposes has historically not qualified as contraband.

This fourth authority may have been operationalized on April 21 when U.S. Navy SEALs boarded the Botswana-flagged oil tanker M/T Tifani in the Bay of Bengal, thousands of miles away from the conflict. The Tifani boarding implicates both Category 4 (contraband) and Category 5 (OFAC sanctions), but the Pentagon appears to have never clarified the legal authority for the boarding. (In theory, it could fit under a stretch of Category 2, but that blockade is geographically limited to the Gulf of Oman and Arabian Sea.)

Setting aside the sanctions question — addressed in Category 5 below — the contraband theory itself is legally shaky on the facts. The Tifani was allegedly carrying Iranian oil that was transiting away from Iran, and appears not to have been “destined for an enemy.” The sanctioned crude oil tanker had four different names, and its current flag was listed as “not known.” It was therefore likely a vessel “without nationality” within the meaning of Article 110 of the U.N. Convention on the Law of the Sea. Stateless vessels present an independent authority to “visit” and determine their nationality. But this is a peacetime—not wartime— authority that is far more constrained.

The U.S. military has since boarded three more Iranian-flagged vessels off the Malaysian and Indian coasts—thousands of miles away from Iranian waters. Soon thereafter, the Pentagon announced that “[i]nternational waters are not a refuge for sanctioned vessels”—but under what authority did the Navy SEALs board, peacetime or wartime?

Further complicating matters, for Weapons of Mass Destruction (WMD)-specific interdictions beyond the immediate conflict zone, the Proliferation Security Initiative (PSI) provides a separate legal framework. Those bilateral agreements—negotiated separately with individual flag states—are where the PSI’s real boarding authority lives. The United States has concluded such agreements with several major flag-of-convenience registries, including Panama, Liberia, the Marshall Islands, and the Bahamas. Where those agreements are in force, they authorize boarding to seize WMD-related delivery systems and materials without requiring separate flag state consent.

The PSI framework is further bolstered by U.N. Security Council Resolution 1540, which affirms that the proliferation of “nuclear, chemical, and biological weapons, as well as their means of delivery, constitutes a threat to international peace and security.”

In sum, some interdictions under this category appear to rely on peacetime rather than wartime authority.  The designation of oil as contraband is contested absent proof of destination to the enemy and for a military purpose, and WMD-specific seizures beyond the conflict zone implicate a separate framework under the PSI and bilateral flag-state agreements.

Category #5: Global “Blockade” of Sanctioned Vessels 

Finally, the assertion of a belligerent right to visit sanctioned vessels of any flag state anywhere in the world is remarkably broad and lacks a sound legal basis in international law. Before the Iran conflict, in December 2025, the U.S. Coast Guard boarded and seized “The Skipper” off the coast of Venezuela, a peacetime maritime law enforcement operation expertly unpacked by Coast Guard Commander Elizabeth Hutton. The Skipper was a stateless vessel. The “visit” of the vessel was lawful as a maritime law enforcement operation, although questions remain about follow-on jurisdiction over the Skipper.

Category 5, as set forward by the Administration, is problematic.

No established doctrine treats domestic sanctions designations as a trigger for belligerent boarding authority. A domestic Treasury Department administrative designation alone cannot create a belligerent right to board under the law of naval warfare, but that is exactly the authority asserted. The reason is straightforward: no naval warfare manual speaks directly to the applicability of Treasury sanctions as a belligerent right to board vessels. Indeed, both the San Remo Manual and the Commander’s Handbook on Naval Warfare are silent on the treatment of domestically sanctioned vessels. We are, literally, in uncharted legal territory.

The 2023 Newport Manual on the Law of Naval Warfare further distinguishes between blockades under the law of naval warfare from “embargo or sanction enforcement operations” that fall under a different legal framework.  To be sure, there can be a belligerent right to board neutral vessels that are resisting visit and search, concealing their identity (such as shadow fleets that use spoofing technology to do so), and vessels that are attempting to breach a blockade. But the authority to board vessels in violation of OFAC sanctions rests squarely on law enforcement authorities. The United States cannot board a neutral vessel in the Pacific solely in violation of domestic OFAC designations without the consent of the flag state or an independent legal basis.

This distinction between law of naval warfare boarding authorities and law enforcement boarding authorities is important, as peacetime Maritime Interception Operations (MIO) operate under a distinct and more constrained legal standard. MIO is defined under joint U.S. military doctrine as “efforts to monitor, query, and board vessels in international waters to enforce sanctions such as those in support of United Nations Security Council Resolutions and/or to prevent the transport of restricted goods.” In this fifth category, the U.S. Navy appears to be overstating its blockade authorities to encompass vessels that are solely sanctioned by the Department of the Treasury.

Outside the law of naval warfare, the right of visit under UNCLOS Article 110 authorizes warships to board foreign vessels on the high seas only in a narrow set of circumstances: piracy, slave trade, unauthorized broadcasting, statelessness, or when the vessel is of the same nationality as the warship. Sanctions evasion – let alone domestic sanctions evasion – is unsurprisingly absent from that list. Bilateral flag state agreements—ship-boarding agreements negotiated under PSI and other frameworks—provide independent authority. That argument works when those agreements exist and are operative. It fails when they do not, or when the flag state objects.

For example, a sanctioned vessel transporting oil to a Chinese refinery in the Pacific Ocean that has not accepted U.S. jurisdiction may not be violating any law that applies to it. When the U.S. military boards that vessel at gunpoint in the Indo-Pacific, it is not enforcing a belligerent right. It is unilaterally extending U.S. domestic administrative law into international waters through military force, which is a very different—and a far more legally contested—proposition. (As noted above, Category 2 would not apply here as that blockade is geographically limited to the Gulf of Oman and Arabian Sea.)

The Five “Blockades”

Conclusion

Iran has called the U.S. military’s Touska seizure piracy. That characterization is wrong under international law—but the legal distance between a lawful wartime capture and something courts might instead find unlawful is shorter than one might expect, especially when enforcement occurs thousands of miles from the declared blockade zone. The Tifani boarding may prove the most legally consequential naval enforcement action of this conflict, precisely because it was the quietest: no declared zone, no announced authority, just Navy SEALs boarding a Botswana-flagged tanker in the Bay of Bengal on the troubling theory that a Treasury Department designation confers a belligerent right to board.

It does not, as a matter of established international law. And the five maritime enforcement operations analyzed here do not share equal legal footing. Iran’s blockade of the Strait of Hormuz (Category #1), indiscriminate mining, and attacks on neutral vessels are clearly unlawful. The core blockade of Iranian ports (Category #2) and the interdiction of contraband destined for Iranian military use (Category #4) appear to rest on solid foundations in the law of naval warfare. The interdiction of all Iranian vessels (Category #3) similarly draws on well-established belligerent rights. But the assertion of a belligerent right to board vessels carrying active OFAC sanctions (Category #5) conflates domestic administrative law and law enforcement with the law of armed conflict. A Treasury Department designation is not a predicate for wartime boarding authority. It never has been. And no naval warfare manual recognizes it as such. If accepted, this theory would mark a significant and unilateral expansion of maritime enforcement authority under international law.

This distinction matters beyond the immediate conflict. If the United States normalizes the theory that OFAC sanctions independently authorize military boarding anywhere in international waters, it will have unilaterally extended domestic administrative law into the high seas through force.

The Pentagon should clarify, with precision, the legal authorities underpinning each maritime enforcement action. Is the boarding an assertion of the law of naval warfare or a peacetime Maritime Interception Operation? It cannot coherently be both. Getting that legal characterization right is not a technicality. In a conflict where Iran’s own blockade is illegal, and the ceasefire is simultaneously purportedly in effect, the United States has every reason to ensure that its own enforcement actions rest on unimpeachable legal ground. If the United States intends to rely on wartime authorities, it must say so clearly. If it does not, it risks reshaping the law of the sea not through agreement, but through ambiguity backed by force. That is not a steady equilibrium that will serve long-term U.S. or global interests.