Maritime coercion sits on a narrow legal ledge: the same boarding and rerouting that look like unlawful interference in peacetime may be a lawful blockade if — and only if — the demanding conditions of the law of naval warfare are actually met.
The Short Version
- CENTCOM says it has redirected roughly 100 commercial vessels while enforcing a naval blockade aimed at constraining Iranian maritime commerce, with humanitarian carve-outs and limited boardings.
- The core legality turns on classification: wartime blockade under the law of armed conflict versus peacetime interference with transit and commerce under the law of the sea.
- Iran and several commentators call the U.S. campaign an unlawful use of force under the UN Charter; other experts argue Iran’s own closure measures breached transit-passage rules, strengthening the U.S. self-defense frame.
- Operational lawfulness requires a declared, effective, impartial, and proportionate blockade that respects neutral and humanitarian traffic — criteria that can be assessed from observable practice.
What CENTCOM says it is doing — and why that distinction matters
U.S. Central Command has stated that its forces have redirected about 100 commercial ships in enforcing a blockade against Iran — a metric used to signal effectiveness and discipline rather than generalized disruption. The command has described limited boardings and vessel disables alongside exemptions for humanitarian shipments, presenting the operation as a law-of-war–compliant blockade targeting Iranian maritime revenue streams, not an indiscriminate throttling of Gulf trade. The label is consequential. If this is a belligerent blockade arising from an armed conflict and executed within the law of naval warfare, then redirection and limited interdiction can be lawful. If instead this is peacetime coercion in a strait and approaches to Iranian ports, many of the same acts risk violating the UN Charter and rules of the sea.
Why the hinge? Because international maritime law splits along two bodies of rules with different permissions and constraints: the law of the sea governs navigation in peacetime, including transit passage in international straits; the law of naval warfare governs belligerent rights and obligations once an international armed conflict exists. The same boarding ladder is judged by different yardsticks depending on which body of law applies.
The legal architecture: transit passage versus belligerent blockade
Start with transit passage. The Strait of Hormuz functions as an international strait; ships not party to hostilities enjoy a right of unimpeded transit passage that coastal states may not suspend. That proposition sits in UNCLOS and customary law and has been repeatedly articulated by maritime-law scholars: closing or materially hampering neutral shipping through Hormuz violates those rules. Against that backdrop, Iran’s threatened and, at points, enforced closure measures — from warnings to attacks and mines — are widely assessed as unlawful interference with transit passage for neutral shipping. Those Iranian measures matter to the U.S. legal narrative because an unlawful obstruction that threatens neutral navigation strengthens the plausibility of responding under the separate body of law that governs armed conflict at sea.
Enter the blockade. Classic naval-warfare doctrine, as consolidated in the San Remo Manual and reflected in state practice, allows a belligerent to declare and enforce a blockade if stringent conditions are satisfied: formal declaration and notification; effectiveness (it must actually prevent access); impartiality (applied without discriminating among neutral flags beyond the objective scope); proportionality (the expected concrete military advantage must not be outweighed by excessive harm to civilians); and respect for humanitarian and neutral rights. Modern experts emphasize those elements as the central test of lawfulness. A blockade that threads those needles is not, by definition, a peacetime “interference with navigation”; it is a regulated method of economic warfare authorized in an international armed conflict.
The competing cases: where the disagreement is real
Iran’s position is categorical: the U.S. campaign is a gross violation of sovereignty, a breach of Article 2(4) of the UN Charter’s prohibition on the use of force, and an act of aggression absent Security Council authorization or a valid claim of self-defense. Iranian officials further condemn interdictions and crew detentions as escalation beyond any plausible legal basis. Moscow has echoed the Charter-based critique, casting U.S. measures as unilateral and unlawful. Several legal commentators, writing from the jus ad bellum lens, agree that a blockade not rooted in Security Council authorization or a narrowly tailored self-defense claim violates the Charter, regardless of compliance with technical blockade rules. The thrust is simple: label this peacetime; everything that follows is illegal.
The contrary view begins from Iran’s own conduct. Maritime-law experts argue that Iran’s obstruction of neutral passage and coercive “closure” measures lack a legal foundation under the law of the sea, while international humanitarian law does not categorically prohibit economic warfare once an armed conflict exists. Under that analysis, a blockade may be used to suppress unlawful interdiction, provided it meets the strict blockade conditions and preserves humanitarian and neutral shipping — which CENTCOM claims to be doing through exemptions and measured enforcement. The legal fulcrum here is not preference but classification: if the situation has crossed the armed-conflict threshold and the blockade meets the San Remo–style requirements, enforcement actions like redirection and limited boardings fit within recognized belligerent rights.
How to evaluate lawfulness from practice, not labels
Set aside rhetoric and look for observables that map to the rule set. First, declaration and notification: lawful blockades are announced, geographically specified, and publicly communicated to mariners. Second, effectiveness: raw counts of redirected vessels — the “100 ships” milestone — are relevant only insofar as they show the blockade is neither fictitious nor geographically overbroad in practice. Third, impartiality: consistent, criteria-based enforcement against carriage to or from Iranian ports, regardless of flag, is required; selective pressure against disfavored states would undercut legality. Fourth, proportionality and humanitarian passage: explicit carve-outs and documented safe passage for medical supplies, foodstuffs, and relief consignments weigh in favor of lawfulness; generalized economic pain to neutrals that is not tied to a concrete military advantage cuts the other way. These are not abstractions; each can be audited by shipping advisories, Notices to Mariners, AIS route patterns, and reported inspection outcomes.
Finally, geography matters. Experts have underscored a key distinction: blockading Iranian ports is not the same as blockading the Strait itself. A strait blockade would predictably impede access to other Gulf states’ ports and threaten neutral transit passage — making it far harder to justify under any body of law. By contrast, a port-focused blockade, if tightly bounded and effectively notified, has a clearer path to legality while still exerting pressure on Iran’s maritime revenue. The operational footprint will reveal which of these the U.S. is actually doing.
Strategic consequences beyond the courtroom
Even a technically lawful blockade is not costless. Energy trades routed through Hormuz are exquisitely sensitive to perceived risks; disruptions and re-routing drive insurance premia, alter tanker economics, and ripple into global prices. Analysts have tied past closures and quasi-closures to significant fractions of global crude and LNG flows — the kind of macro exposure that turns narrow legal determinations into system-wide effects. A blockade that remains disciplined, port-focused, and time-bounded can narrow contagion; an operation that creeps into the strait or unpredictably escalates interdictions will transmit volatility far faster than lawyers can parse footnotes.
The upshot is clear. The legality of the U.S. redirection campaign does not turn on who shouts “illegal” or “lawful” the loudest; it turns on whether the facts on the water align with the law’s demanding checklist for a blockade, and whether Iran’s own unlawful interference with neutral transit altered the governing frame from peacetime navigation law to the law of armed conflict. On present, attributable statements, CENTCOM’s milestone and framing place the operation within the blockade box, with the burden now on sustained practice — notifications, humanitarian corridors, impartiality, and proportionality — to keep it there.
⚡ DECIS ALERT — OCCURRED: US redirects 100 ships during Iran blockade
US Military Redirects 100 Ships During Iran Blockadehttps://t.co/lgSuuRDqbc pic.twitter.com/hde1ZG0m5C
— Decis Intelligence (@decisintel) September 12, 2026
How to read the next update
When the next “redirected ships” tally arrives, ignore the number in isolation. Ask instead: Is the declared blockade area still limited to Iranian ports and approaches, or bleeding into strait transit lanes? Do humanitarian consignments continue to move? Are neutrals treated consistently? Are interdictions tied to a specific, articulable military advantage, not broad commercial strangulation? Those are the markers that will decide whether this remains a legally sustainable belligerent blockade — or slides into the category its critics allege.
Sources:
insiderpaper.com, centcom.mil, iranintl.com, arabnews.com, tass.com, penningtonslaw.com, npr.org, justsecurity.org, thenation.com
