Both sides in the war between the United States and Iran are now routinely attacking oil tankers, and both describe the other’s attacks as unlawful while defending their own. It is worth setting out what the law of armed conflict actually says about merchant ships, because the answer is more permissive than most readers expect and more constrained than either government’s rhetoric suggests.

Nothing here resolves who is right in the current fighting. It is a description of the framework the arguments are being made within.

The starting position: civilian objects are protected

The basic rule of targeting is distinction. Attacks may be directed only at military objectives, meaning objects which by their nature, location, purpose or use make an effective contribution to military action, and whose destruction offers a definite military advantage.

A merchant ship carrying commercial cargo, crewed by civilian seafarers, is presumptively a civilian object. Its crew are civilians. The presumption is the default, and the burden of displacing it falls on the party attacking.

How the presumption can be lost

Naval warfare has long recognised that a merchant vessel can become a lawful target through what it does. The most widely cited modern restatement is the San Remo Manual on International Law Applicable to Armed Conflicts at Sea, produced by legal and naval experts in 1994. It is not a treaty and binds nobody by itself, but states and their legal advisers treat it as an authoritative summary.

The manual lists circumstances in which an enemy merchant vessel may be attacked. They include carrying military materiel, acting as an auxiliary to enemy armed forces, being incorporated into or assisting an enemy’s intelligence system, sailing under armed convoy escort of enemy warships, refusing to stop when ordered or actively resisting visit and search, and otherwise making an effective contribution to military action.

Three features of that list are worth noting. It is about conduct and use, not ownership. Being registered to an enemy state is not, on its own, sufficient. And it distinguishes enemy merchant vessels from neutral ones, which enjoy stronger protection still — a neutral vessel generally becomes targetable only through conduct such as breaching a lawfully established blockade after warning, or carrying contraband.

The crew problem

Even where a vessel is a lawful target, the rule of proportionality still applies. An attack expected to cause incidental civilian death or injury excessive in relation to the concrete and direct military advantage anticipated is prohibited, and feasible precautions must be taken to minimise harm.

Seafarers are the civilians most exposed by a shipping war and the least represented in the argument about it. They do not choose the routes, they frequently do not know the beneficial ownership of the cargo or the vessel, and they are often nationals of countries entirely uninvolved in the conflict. A ship becoming a lawful target does not make its crew combatants.

There is also a duty to take measures for the safety of survivors after an engagement at sea, so far as military circumstances permit. In practice this is one of the least observed obligations in naval conflict, because rescuing survivors requires stopping in a contested area.

Where the current arguments sit

Each side’s legal case can be stated in this framework, and each has an obvious weak point.

The United States has characterised Iranian tankers as sustaining a war effort and its own strikes as responses to attacks on warships and shipping. The weak point is that a tanker’s cargo funding a state is a long way from the direct contribution to military action the rule requires — otherwise any export from any belligerent would be targetable, which would leave the category of civilian object with almost nothing in it.

Iran has invoked Article 2(4) of the UN Charter and called the strikes war crimes, while itself warning that vessels transiting the Strait of Hormuz without its authorisation will be targeted. The weak point is that no state may condition passage through an international strait on its own permission, and threatening neutral commercial shipping for non-compliance is difficult to defend under any reading of the framework above.

A further point, often lost: Article 2(4) governs whether resorting to force is lawful, while the targeting rules govern how force may be used once fighting has begun. They are separate bodies of law, and a party can be wrong on one and right on the other. Arguments that mix them usually do so deliberately.

Our report on the weekend’s exchanges is here.

Sources

• San Remo Manual on International Law Applicable to Armed Conflicts at Sea (1994), provisions on enemy and neutral merchant vessels
• Additional Protocol I to the Geneva Conventions (1977), rules on distinction, military objectives and proportionality
• Charter of the United Nations, Articles 2(4) and 51
• United Nations Convention on the Law of the Sea, provisions on transit passage through international straits
• International Committee of the Red Cross customary international humanitarian law study
• Statements by the US Department of Defense, CENTCOM, the IRGC Navy and Iran’s foreign ministry, September 2026