How the US Navy could blockade Iran’s ports and sweep mines from the Strait of Hormuz

By Brad Lendon, CNN

(CNN) — Six weeks after the beginning of the war with Iran, President Donald Trump is giving the United States Navy its most difficult jobs of the conflict: blockading Iran’s ports and clearing the strategic Strait of Hormuz of any Iranian mines.

The blockade order would apply to all Iranian ports, both inside and outside of the strait –– a critical passage for the global energy trade on which Iran has had a chokehold since the war broke out – from 10 a.m. ET Monday, US Central Command (CENTCOM) said.

Trump indicated that mission would have an even wider scope too, possibly well outside the Persian Gulf.

“I have also instructed our Navy to seek and interdict every vessel in International Waters that has paid a toll to Iran. No one who pays an illegal toll will have safe passage on the high seas,” he said Sunday, referring to Tehran’s move to charge ships for safe passage.

The point of the mission is to maximize pressure on Iran by strangling its cashflow from the energy trade. But solving the global energy crisis this war will take another tough job: clearing any sea mines Iran has laid.

On Saturday, Trump said the Navy had begun minesweeping operations in the strait. CENTCOM affirmed that, saying two US guided-missile destroyers had entered the strait to begin “setting conditions for clearing mines.”

The missions mark a shift in this conflict, from the skies to the sea. To date, the conflict has been mostly conducted by air, although a US submarine did sink an Iranian navy frigate off Sri Lanka in the early days of the war.

Navy aircraft flying off aircraft carriers have also been involved.

But those missions aren’t as complex, or as risky, as what Trump is asking of the Navy now.

Here’s a look at what’s involved.

What is a blockade?

A blockade is a tool of economic warfare as much as it is kinetic warfare.

The Newport Manual on the Law of Naval defines a blockade as “the capture of contraband, and the capture or destruction of enemy property found at sea.”

“These methods deny an enemy the chance of economic revenue from its exports and the benefits of imports that support its war effort,” the manual says.

To be legal, the imposition of a blockade must follow certain rules, including:

• It must be declared and notified, meaning warnings must go out to ships that it might affect.
• It must be effective, meaning the US must have the ships and aircraft to enforce it.
• It must be impartial, affecting the vessels of any nation.
• It cannot be targeted solely at civilian populations, but harm to civilians is acceptable.
• It must not block access to neutral ports and may not block a strait, like Hormuz, which Trump has said is open to non-Iran- related international shipping.

Can the US effectively pull it off

Closing off Iran’s ports, almost all of which are inside the Strait of Hormuz, to oil tankers and other merchant vessels would be “procedurally difficult, but practical if the US has maritime superiority,” said analyst Carl Schuster, a former US Navy captain.

And that may not be the case.

Iran still has the ability to fight back with mines, an unknown number of small boats that can carry missiles, surface drones, aerial drones and land-based cruise missiles as well as shoulder-fired anti-aircraft missiles that could target helicopters and fighter jets protecting the ships in the water, analysts say.

Yu Jihoon, a research fellow at the Korea Institute for Defense Analyses and a former South Korean submarine officer, called the blockade “high risk” because of those Iranian options to strike back.

“If Iran accepts it as a violation of its sovereignty or a de facto expansion of maritime warfare, the possibility of a local military conflict could increase,” Yu said.

James Stavridis, a retired US Navy admiral, told CNN’s Fareed Zakaria, that he thinks the Pentagon would need two aircraft carrier strike groups and about a dozen surface ships outside the Gulf to patrol the Strait of Hormuz at its entrance.

Inside the Gulf, Stavridis said at least six US destroyers would be needed, along with help from the navies of American partners like the United Arab Emirates and Saudi Arabia.

“You want to try to bottle it up on both sides,” he said of the strait.

Schuster said the US Navy trains boarding teams of about 10 to 14 people to take control of merchant ships. Each team includes an “officer of the deck” who essentially acts as the merchant ship’s captain after a takeover and “guides it to an anchorage or port for detainment.”

But all that takes time.

Schuster says of six US destroyers inside the strait, two will be used to do boardings, with the other four nearby to deal with any Iranian attempts to stop those actions.

The two destroyers could possibly seize six ships a day between them, Schuster said.

Prior to US and Israel’s war on Iran, some 130 ships a day were passing through the strait, through which roughly a fifth of the world’s oil and gas flowed.

What is ‘prize law?’

Jennifer Parker, a non-resident fellow at the Lowy Institute and former Royal Australian Navy officer, said the seizure approach is the more likely option for the US to use in trying to block Iranian shipping.

Parker said it falls under international “prize law.”

According to the Newport manual, “belligerents at sea” can capture enemy merchant vessels and goods outside neutral waters. They can also subject “neutral” merchant vessels to visit, search, diversion, and apply capture “if they carry contraband.”

Prize law also states that neutral merchant shipping anywhere can be attacked as military objectives if they “make an effective contribution to the enemy’s military action or war-fighting.”

So rather than a blockade (as stated), what we are more likely to see is selective interference with shipping under prize law to influence shipping routes, reduce Iranian control & generate economic leverage,” Parker wrote on X.

Historically blockades were implemented close to a nation’s shores, but modern intelligence, search and reconnaissance makes longer-range operations feasible, said Alessio Patalano, professor of war and strategy at King’s College London.

It’s also possible to begin the operations farther from Iran, then move closer as conditions warrant, he said.

This would prevent Iran from immediately bringing its advantages of small craft and short-range weaponry into play, he added.

Mines and minesweeping

Shortly after the war began, two people familiar with US intelligence told CNN that Iran had begun laying a small number of mines in the Hormuz strait.

Two US destroyers – the USS Michael Murphy and USS Frank E. Peterson – went through the strait over the weekend, but Schuster said they were unlikely to be doing any actual mine clearance, and they are not the prime platforms for that work.

More likely, he said, is that the destroyers went through the strait to demonstrate that such navigation was possible and that there were no mines there.

The actual minesweeping work is more likely to be done by underwater drones, littoral combat ships equipped with a mine-countermeasures package and helicopters, Schuster said.

Mines come in many different forms, he said, and some may not have been detected by or set off by the US warships.

Among those Iran could deploy in the strait are:

• Spiked contact mines like those seen in World War II movies.
• Influence mines that are set off by the static electricity ships generate when moving through salt water.
• Magnetic mines that react to changes in the “magnetic signature” of the water when ships pass through it.
• Acoustic mines that react to noises the ships make as they pass over.
• Pressure mines that detonate when water pressure changes to an amount the mines measures as from a type of ship it is designed to destroy.

Some complex mines contain combinations of the types above, making them particularly hard to counter, Schuster said.

And some advanced mines have counters that will let a certain number of ships pass before detonating.

“These mines make it very difficult to determine if all the mines in a minefield have been detonated or otherwise neutralized,” he said.

Mines are countered in two key ways, sweeping and hunting, Schuster said.

For moored mines, sweeping uses mechanisms that cut cables that attach the mines to the sea floor. The mines will then float to the surface where they can be destroyed.

For bottom mines, minesweeping ships tow gear that can mimic the acoustic, electrical or magnetic signatures of ships and detonate them safely.

But sweeping techniques don’t work against complex and pressure mines, according to Schuster.

They can be detected by sonar on underwater drones or lasers mounted on the drones or even on helicopters and then destroyed safely.

Analysts also note that US minesweeping capacity alone is limited.

The US Navy decommissioned its four specialized minesweepers that had been based in Bahrain, in the Persian Gulf, last year.

Minesweeping duties were turned over to three littoral combat ships equipped with the Mine Countermeasures package, but the location of those ships has not been disclosed. Two of them were seen in Singapore last month.

Analysts said Washington may have to look outside its own ranks to get a thorough sweeping of mines in the Strait of Hormuz.

“This is an area in which the US Navy would probably rely on allies and partners more than one would assume,” Patalano said.

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