The sinking of Iran’s Iris Dena in the same week as a Russian oil tanker going down to Ukrainian USVs provides a useful point of comparison. The two capabilities are often positioned as opposing methodologies: The antiquated submarine vs the new modern USV. The reality is much more nuanced. 

By Sam Cranny-Evans, editor of Calibre Defence, published on March 5, 2026.

An attack submarine from the US Navy has sunk an Iranian frigate called the Iris Dena. According to statements from the US Secretary of War, the engagement took place late on Tuesday 3 March. The ship was around 80 km off the coast of Sri Lanka as it made its way back to ports following an international exercise with India. 

The video released by the US shows a single Mk 48 heavyweight torpedo hitting the rear of the ship. The Sri Lankan coastguard received a distress call at 5.08am on Wednesday morning, and when they arrived a few  hours later, the ship had sunk. At least 87 Iranian sailors are confirmed to have died, and 32 were rescued out of a crew of 140. 

Not long before the torpedo hit the Iris Dena, a Russian tanker called Arctic Metagaz caught fire and exploded. It was 240 km off the coast of Libya when it eventually sank. The crew of 30 were saved, but Putin and other Russian figures have accused Ukraine of sinking the ship. Without evidence, they have claimed that two Ukrainian uncrewed surface vessels hit the Arctic Metagaz. It was reportedly on its way to an Egyptian port when it sank. 

What does the sinking of the Iris Dena have to do with Arctic Metagaz?

Well, we are seeing two different methods of fighting a war play out in real time. There is a growing set of voices within defence that argue against reliance on conventional capabilities. ‘A submarine is too big and expensive, you can’t build enough of them,’ they might say. 

Similar arguments are played out for tanks, fast combat aircraft, and somehow for air defence. Drones and uncrewed platforms are the future, these voices will often argue. And certainly, the sinking of the Arctic Metagaz, as well as several Russian vessels in the Black Sea, suggests that USVs can be a problem. 

But the sinking of the Iris Dena should stand as a reminder that conventional capabilities are phenomenally lethal, too. While a USV strike tends to rely on holing a ship and causing internal damage, the single torpedo that hit the Iris Dena likely caused catastrophic damage immediately. 

There was no need for the submarine to maintain communications links with any other platform. And, in all likelihood, the Iris Dena crew had no idea it was there. In addition, if there were more target ships in the area, the US submarine could have continued its hunt, further degrading the adversary. USVs in contrast, can be countered with watch personnel and the weapons a ship carries. Russia has also used helicopters to defend its ships against them with some success. 

They require a lot of planning and knowledge of the route a ship will take, as well as coordination. Not that a submarine does not need that, but a single SSN with permissions to hunt can effectively coordinate its own actions. Both capabilities are clearly effective and could have their place. The key differentiator seems to be resource demand.

Calibre comment: ASW vs C-USV

And this, I think, is the big difference: Resource demand. When a Russian submarine came too close to a US carrier in 2025, the UK, US and other allies flew constant orbits of their P-8 Poseidon ASW aircraft to try and find it. A British Type 23 frigate is always performing the role of an anti-submarine patrol in the North Sea. And the UK is exploring an expansive and ambitious capability under Project CABOT that is almost dedicated to finding submarines. 

In short, countering a submarine is very difficult. Modern types can be exceptionally quiet and hard to find. Their torpedoes and missiles are difficult to intercept, especially if launched as a salvo. At the same time, finding a submarine takes a lot of resources, resources that few forces in the world have. Altogether this makes a submarine uniquely dangerous in the anti-ship role. 

• Project CABOT, transforming ASW for the Royal Navy – Calibre Defence 
• Russia and the strategic challenge to the North – Calibre Defence 
• Type 26; the right choice for Norway’s ASW – Calibre Defence 

By contrast, the counters to a USV require good situational awareness, and the weapons on board a ship that are effective against them. A single ship’s crew would theoretically be capable of defending itself against a USV attack. Provided of course that it has the right resources. But it is unlikely that a single Type 26 ASW frigate would be able to track and destroy a Yasen class submarine. 

In sum, countering a submarine likely imposes very different resource demands on a force to USVs. And a submarine is also likely to be more effective once it gets within range of a target. However, USVs can add noise and confusion, and prove useful in hitting unprepared targets at long range. They are not an idle threat to modern naval power. But their utility should be balanced with the much greater lethality of a submarine. 

The lead image is a screen grab from the DoW’s video showing the sinking of the Iris Dena. Credit: US DoW.