Hours after Donald Trump declared a naval blockade, multiple tankers emerged from Iranian ports and left the Strait of Hormuz, testing the resolve of US forces waiting on warships in the Gulf of Oman.
The response of the US military — given orders to enforce the blockade, itself an act of war — will represent another pivotal moment in a weeks-long conflict that has become a battle to control the world’s most important energy waterway.
The US operation requires a host of naval, air and intelligence resources, and raises complex questions around when and where to board ships — and what to do with them afterwards. The US deployed many of these tactics in a recent partial blockade of Venezuela.
The US military has committed a substantial force to the blockade, with more than a dozen warships, over 100 fighter jets and surveillance aircraft, and upwards of 10,000 troops enacting it.
But there are just eight days to go in Washington and Tehran’s two-week ceasefire, and it could take longer for Iran to feel the economic pain from a slowdown in oil exports, experts said.
“Blockades rarely achieve an outcome in any short period of time, historically. They take time to have weight,” said retired vice-admiral Kevin Donegan, former deputy chief of naval operations.
The US military has said it would block “all maritime traffic entering and exiting Iranian ports” and coastal areas, while letting through commercial vessels servicing non-Iranian ports. The blockade would be enforced in the Gulf of Oman and Arabian Sea east of the strait, via “interception, diversion and capture”, it said.
The blockade is aimed at exerting pressure on the Iranian regime after a first round of negotiations failed to reach an agreement, former military officials and analysts said.
By Tuesday afternoon, the blockade appeared to be influencing ships’ movements: a sanctioned tanker travelling through the strait had U-turned and others that passed through had subsequently stopped, ship-tracking data showed.
US Central Command, which oversees American military operations in the region, said no ships coming from Iranian ports went through the US blockade in its first 24 hours and six commercial vessels turned around when ordered to do so.
The decision to operate in the Gulf of Oman and Arabian Sea allows US forces to stay as safe as possible: “There’s no reason to have that confrontation in someone’s backyard if you can do it well clear” of there, Donegan said.
Among the American warships enforcing the blockade are an aircraft carrier, an amphibious assault ship and destroyers. The effort also requires fighter jets such as F-35s, F-18s, F-16s or F-15s flying above the US ships, to shoot down incoming drones or cruise missiles. The planes are taking off from ships and bases.
The first step is to identify offending ships using surveillance from drones and satellites, as well as destroyers’ radars. Once a ship has been deemed suspicious, the Navy would hail it via radio and tell it to stop, said Mark Cancian, a former Pentagon official.
Ships that do not turn around would be intercepted by visit, board, search and seizure (VBSS) teams from either the Navy, Coast Guard, Marine Corps or special operations forces.
The VBSS team — made up of about six to 16 personnel — would likely approach in a helicopter, hover over the tanker, abseil or rappel down and fan out over the ship. US forces followed this procedure when recently seizing at least 10 vessels coming from Venezuela.
Most vessels would likely comply with a request to board from the US Navy, but some might not, said Donegan. In those cases, the US would send more highly trained VBSS teams, such as Marines or special forces, he added.
Some former admirals and analysts said Marines from the 31st Marine Expeditionary Unit, which is on the amphibious assault ship USS Tripoli, would suit missions to board, stop or divert a ship well. The USS Tripoli, which is part of the blockade, can carry about two dozen aircraft, including F-35s, V-22 Ospreys and helicopters.
Once US troops have boarded a ship, they can inspect the cargo and the vessel’s records to see where it has been, where it might be going and where goods on board came from, said Donegan.
If there is evidence the ship was going from or to an Iranian port, or carrying Iranian cargo, the US would then have to decide what to do with the vessel — at present the most opaque element of the US plans for the blockade.
The first boarding and seizure will “give us a lot more clarity about what this blockade will be like”, said Cancian.
One option is “catch and release” — interdicting the tanker, turning it around and telling it to go elsewhere, said Joshua Tallis, an analyst at the Center for Naval Analyses.
The US could also seize the vessel and hold it in a local or neutral port with permission from that third country. A third possibility is seizing the cargo, selling any oil on board, and eventually releasing the ship, as the US did with some tankers travelling from Venezuela.
However, seizing a vessel could present complex legal issues, said retired vice-admiral John Miller, who commanded US naval forces in the Middle East: Any nation that “accepts those ships then becomes a belligerent in the conflict”.
Arsenio Dominguez, secretary-general of the International Maritime Organization, said on Monday that the entire blockade runs counter to maritime law. “Countries don’t have the right to blockade an international strait,” he told journalists.
Miller said “most ships will be deterred and opt not to go to Iran”, but “black and grey-market ships are most likely to try”.
They may attempt to overwhelm US ships by moving in large numbers, as seen in Venezuela last year. “Groups of tankers fled all at once, frustrating US efforts to catch them. The Iranians may attempt something similar,” said Gregory Brew, a senior analyst at Eurasia Group.
However, the US does not need to intercept every ship potentially carrying Iranian goods for the blockade to be effective, said retired rear admiral Mark Montgomery. US forces would just “need to seize enough of them” to deter others from trying to make the journey, he said.
“There are always going to be some ships that get through,” said Cancian.
Windward, a maritime AI company, tracked two other tankers loading at Kharg Island, Iran’s vital oil export hub, on Monday, with an estimated combined capacity of 4mn barrels. Windward also detected three dark vessels likely loading at Assaluyeh Port and 14 dark tankers in the port’s waiting area.
Enforcing the blockade away from the strait and Iranian coastline reduces the risk of US naval ships being attacked by Iran’s fast boats, drones and cruise missiles — but there is still a risk of strikes.
Iran has also said it would target the ports of US allies. Should Iran follow through on those threats, it could escalate and “become a break in the ceasefire”, said Donegan.
The blockade seems aimed to escalate the conflict “in a way that also allows some wiggle room for negotiations to still be happening in the background”, said Donegan. “Those get harder if bombs are flying.”
Additional reporting by Charles Clover in London. Data visualisation by Alan Smith