HOUSTON: The sinking of an Iranian frigate by a US submarine risks an expansion of the war beyond its main theatre in Iran – potentially even as far as South-east Asia, experts say.
The concerns stem from the torpedoing of IRIS Dena by an unidentified US submarine in the Indian Ocean, off Sri Lanka’s southern coast, on March 4.
One of Iran’s most modern warships, the Dena was armed with anti-ship missiles, torpedoes and a helicopter. Commissioned in 2021, it had indigenously built engines and armaments.
US President Donald Trump has declared that destroying the Iranian navy is one of the objectives of the joint US-Israeli war on Iran, which entered its sixth day on March 5.
The risk to South-East Asia depends on “how far the Trump administration wants to take things in terms of hunting down the last remaining Iranian sampan”, said Professor Joseph Liow, chairman of the Middle East Institute at the National University of Singapore.
“There is probably reason to be concerned about the extent to which the US and Israel are prepared to execute their war plans,” he told The Straits Times.
“There is the element of unpredictability not only to war but really to this administration, which complicates any and all scenarios.”
With conflicts, there is always the “escalatory ladder” that risks expansion of the war in terms of intensity, geography and cost, among other things, Prof Liow noted.
And there is a lack of clarity about the US strategic objectives, which is troubling, he said.
“We’ve heard, at different times, that the war is about regime change, but also about the destruction of missile capabilities, taking out once and for all enrichment capacity, crippling the Iranian navy, and about decapitating the leadership. The list goes on.
“Without clarity on strategic objectives, battlefield objectives will also suffer from lack of clarity,” Prof Liow observed. “This raises the risk of expansion of the conflict.”
Gregory Poling, who directs the South-East Asia Programme and Asia Maritime Transparency Initiative at the Center for Strategic and International Studies in Washington, said the incident plays on anxieties about the Trump White House.
“I don’t know if South-East Asian governments will view this as bringing the war closer, but it will reinforce their anxiety about the Trump administration,” he told ST.
“The war is at best legally dubious, and the frigate sinking is yet another example,” he said.
The rules governing warfare are clear-cut. The United Nations Charter bans the use of force except in self‑defence or under UN Security Council authorisation. Under the UN Convention on the Law of the Sea, naval engagements are permitted in international waters if they are tied to an active armed conflict.
The Trump administration, which has not sought authorisation from the UN or the US Congress, has not demonstrated how the frigate presented an imminent attack.
Given the ongoing war, and retaliation by Iran that has included attacks on or an intention to attack US ships, aircraft, bases and embassies, the US holds the frigate to be a legitimate military target.
But critics say the sinking arguably extends the US-Israel-Iran war theatre far beyond the Middle East – Iran is more than 3,000km from Sri Lanka – potentially violating the principles of proportionality and distinction.
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Under the law of armed conflict, attacks must be proportional: Military gain must not be grossly excessive compared with the anticipated harm, for example, to life, the environment and regional stability.
The other core principle is that of distinction: By striking an Iranian warship in a busy area that is not a declared war zone near regional trade routes, the US blurs where the “battlefield” begins.
Even if the target is legitimate, such a move risks escalation, collateral damage to shipping, and the possibility of neutral states being dragged into the conflict.
The incident marked the first time that an American submarine had used a torpedo to sink an enemy ship in combat since World War II, and the first submarine attack of any kind since 1982, when the British navy sank an Argentine cruiser off the Falkland Islands.
The sinking shows the broad scope of how the US military views Iranian targets, said Associate Professor Javed Ali, a national security expert at the Gerald R. Ford School of Public Policy at the University of Michigan.
“This Iranian ship must have been tracked for some period of time. I’m not sure how the approval process came through, or (how) the decision was made to sink it there versus allowing it to go back to Iran.
“But the thought probably was to sink it before it goes back to Iran and assists with the defence of the country,” he added.
US Defence Secretary Pete Hegseth said on March 4 that IRIS Dena was Iran’s “prize ship”, as he briefed the media in Washington.
He described its fate as a “quiet death” for a vessel that “thought it was safe” as it returned from drills in the Bay of Bengal.
The Dena gained publicity in 2022-2023 during a deployment meant to demonstrate Iran’s blue-water naval ambitions and project power into the Indian Ocean.
It had just completed its participation in India’s International Fleet Review 2026 exercises before it was hit.
The Pentagon released infrared video showing a torpedo detonating under the stern, lifting the frigate into the air and breaking its keel. It sank before rescuers from Sri Lanka could arrive on the scene.
Iran, including its ambassador Alireza Delkhosh in Sri Lanka, has issued no public statement. State media outlets such as IRNA and Press TV are also silent.
International reaction has been muted so far. China and Russia, Iran’s strategic partners, have not commented.
Sri Lanka’s government expressed distress over the humanitarian toll – more than 80 dead, 32 rescued, and over 100 missing – while confirming that the incident occurred outside its territorial waters. Avoiding direct condemnation, Foreign Minister Vijitha Herath pledged inquiries.
India, the host of the drills attended by the frigate, has also remained silent. - The Straits Times/ANN
