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News that a United States submarine had torpedoed and sunk the Iranian warship IRIS Dena about 40 nautical miles off Sri Lanka took many observers by surprise. An attack like this so far from the Persian Gulf – and in a key trade route connecting China to the Middle East – suggests the arena of this war may be widening.

But the incident also highlights something rarely well understood outside military and legal circles: the law of naval warfare.

Many have wondered: was this attack lawful? And who was under an obligation to rescue survivors?

When does the law of naval warfare apply?

The law of naval warfare is a subset of the law of armed conflict.

The law of naval warfare sets out permissions and protections for combatants, civilians and neutral actors engaged in conflict at sea.

Importantly, it applies regardless of whether the resort to force was lawful.

So, was it legal for the US to sink the Iranian warship?

Yes, it was a lawful target.

Under the law of naval warfare, warships belonging to a state engaged in an international armed conflict are military objectives by nature. The rules say they may be lawfully targeted.

Such attacks may occur on the high seas or within the 12 nautical mile territorial waters of the states that are party to the international armed conflict (the belligerents). This means, effectively, that such an attack could happen anywhere outside the 12 nautical mile territorial waters of neutral states.

If the Iranian warship was within Sri Lankan waters (that is, within 12 nautical miles of the Sri Lankan coast) at the time, the attack wouldn’t have been lawful.

But in this case, IRIS Dena was reportedly operating outside Sri Lanka’s territorial waters and therefore constitutes a lawful military target.

What does the law say about rescue of survivors?

The law of naval warfare also sets out obligations regarding the rescue of survivors.

Under the Second Geneva Convention of 1949, parties to a conflict must – after each engagement – take all possible measures to search for and collect the shipwrecked, wounded and sick.

These rules apply to naval warfare and require belligerents, so far as military circumstances permit, to assist survivors at sea.

In practice, however, submarines face particular challenges in fulfilling this obligation. Surfacing to rescue survivors may expose them to significant risk. You also can’t usually fit a large number of survivors on a submarine.

If a submarine cannot safely surface to rescue survivors, it may instead facilitate rescue by reporting their location to other vessels or authorities.

This practice has been noted in some key legal commentary on submarine warfare.

The swift response of the Sri Lankan navy, which rescued 32 sailors from IRIS Dena, suggests authorities were informed quickly of the incident. (Sri Lankan officials say 87 bodies were also retrieved).

How Sri Lankan authorities were informed is not yet clear, but it seems likely the US navy transmitted the location of the survivors.

Given the damage suffered by IRIS Dena and the reported casualties, the ship’s crew was unlikely to have been able to transmit their location themselves.

This may also explain why early reports suggested a submarine had sunk the vessel, before the US confirmed its involvement.

It is also unlikely the crew of IRIS Dena would have immediately known they had been struck by a submarine-launched torpedo. Such a torpedo would typically be fired from very far away, beyond the detection range of a ship’s hull-mounted sonar.

A lawful military target

While debate continues over the legal justification for the United States entering the conflict with Iran, the conduct of hostilities at sea is nonetheless governed by the law of naval warfare.

Under that framework, IRIS Dena therefore constitutes a lawful military target, and efforts to facilitate the rescue of survivors are consistent with those obligations.

Header image: In this photo released by Sri Lankan President Media Division, Sri Lankan Navy sailors rescue Iranian sailors from IRIS Dena warship after their ship sank outside Sri Lanka’s territorial waters, near Galle, Sri Lanka, March 4, 2026. (Sri Lankan Presidential Media Division via AP). Credit: AP/ Sri Lankan Presidential Media Division.

This article is republished from The Conversation under a Creative Commons licence. Read the original article. Jennifer Parker is an Adjunct Professor, Defence and Security Institute, The University of Western Australia; UNSW Sydney