The U.S. has destroyed most of Iran’s navy. But not the one Tehran uses to control the Strait of Hormuz.
The regular navy operated Iran’s big battleships largely for prestige and occasional long-range deployments. The paramilitary Islamic Revolutionary Guard Corps, on the other hand, has its own extensive fleet of more nimble boats designed to control the crucial waterway with missiles, mines and harassment of commercial ships—and they are much harder to reach.
Farzin Nadimi, an Iran-focused senior fellow with the Washington Institute, a U.S.-based think tank, said more than 60% of the Revolutionary Guard’s fast-attack craft and speedboat fleet remains intact. They are continuing to pose a threat.
After President Trump struck a deal with Tehran to stop fighting for two weeks in exchange for opening the strait, Iran warned via marine radio that any ships that try to cross without permission from the Revolutionary Guard risk being destroyed.
Only four ships crossed the first day of the cease-fire, the lowest this month, and Iran has told mediators it will limit crossings to about a dozen a day, down from more than 100 before the war.
Iran later issued a warning about antiship mines in the main channel. It told ships to consult with the Revolutionary Guard to steer around them by following new routes along Iran’s coast. It was Iran’s first indication that it may have mined the waterway after the U.S. warned of the possibility last month.
“Their asymmetrical strategy is working,” said David Des Roches, a former director responsible for Persian Gulf policy at the Defense Department.
Two U.S. guided-missile destroyers passed through the strait on Saturday, three U.S. officials said, the first transit by American warships during the war and a challenge to Iran’s control of the waterway.
With a fifth of the world’s oil typically passing through the strait, the shutdown has sent global oil prices soaring past $100 a barrel for the first time since 2022. Iran has controlled traffic by attacking more than two dozen commercial ships in the Persian Gulf. It can continue such attacks without a naval force by launching drones and missiles from land, but military boats allow it to threaten or escort ships directly.
The U.S. has been working to degrade that capability. The U.S. Central Command, responsible for the Middle East, said April 6 it had sunk more than 155 Iranian vessels. Satellite imagery and official U.S. military footage show American strikes have devastated Iran’s naval fleet, including its most sophisticated, powerful models. But most of that damage was inflicted on Iran’s conventional navy.
In one the highest-profile hits, a U.S. submarine torpedoed Iran’s IRIS Dena warship in the Indian Ocean near Sri Lanka with roughly 180 people aboard, at least 87 of whom died. Other successful strikes have included hits on minelayers and frigates.
U.S. strikes also have destroyed some of the Revolutionary Guard’s most advanced combat ships. Centcom footage shows they include the IRIS Shahid Sayyad Shirazi, a stealth catamaran unveiled in February 2024 that could fire antiship and surface to air missiles.
The U.S. also struck the Revolutionary Guard’s biggest drone carrier in the Persian Gulf, the Shahid Bagheri, which was also a launchpad for antiship missiles and helicopters, said Nicholas Carl, a fellow at the Critical Threats Project at the American Enterprise Institute, a Washington think tank.
Four of the Iranian navy’s primary surface combatants—including a Jamaran-class frigate—were likely sunk or crippled by March 5, according to Janes, an intelligence company specializing in military and national security analysis. In total, Iran’s navy has lost six of its seven frigates, its two corvettes and one of three ocean-going conventional submarines, said Alex Pape, head of the maritime team at Janes.
But the Revolutionary Guard still has large numbers of ships designed to harass vessels in the confined waterways of the Persian Gulf and the strait, which at its narrowest is only about 20 miles wide. That is where they are most effective.
The smaller boats are more numerous and harder to spot on satellite than larger conventional vessels, Des Roches said. The Revolutionary Guard has used underground pens hidden along the rocky coast to store hundreds of the smaller attack craft, said Chris Long, a former British navy official in the Persian Gulf.
“It will be a long time before the U.S. can take all those out,” said Long, who now advises shipping companies as head of intelligence at maritime company Neptune P2P.
Iran built out the Revolutionary Guard’s fleet under a change in doctrine it adopted after the U.S. sank much of the country’s active fleet during a one-day naval battle in April 1988. When an American frigate hit a mine during the so-called Tanker War, the U.S. sent in a force to clear out Iranian positions in the Gulf and destroyed Iranian ships that confronted it.
Iran pivoted to an asymmetrical approach focused on controlling commercial shipping, Carl said.
At least 50 Iranian attacks have been carried out against shipping in the Gulf and the Strait of Hormuz since Feb. 28, according to Armed Conflict Location and Event Data, which tracks conflicts worldwide.
The Revolutionary Guard has also begun using waterborne drones to attack ships. The Marshall Islands-flagged Safesea Vishnu tanker was attacked by two explosive-laden drone boats in an Iraqi port on March 11, its owner and operator, New Jersey-based Safesea Group, said.
The MKD VYOM, a Marshall Islands-flagged oil tanker attacked near Oman, and the Bahamas-flagged crude oil tanker Sonangol Namibe anchored near Iraq’s Khor al Zubair port, were also struck by naval drones, according to Ambrey, another maritime security firm.
The Revolutionary Guard first displayed its naval drones about a year ago in a video of an underground base. The Iran-backed Houthi militia in Yemen used drones in 2024 against commercial vessels in the Red Sea, the U.S. military said at the time. Security officials said then that the technology behind the remote-controlled drones had been provided by Tehran.
Mines are another concern, particularly after Iran’s recent warning to commercial shippers to consult the Revolutionary Guard on safe routes through the strait. Their presence hasn’t been confirmed, and some analysts saw the warning as a scare tactic. Still, the notice, which came with a map of a rectangular no-go area in the middle of the waterway, circulated widely between sailors.
Few crew members or captains would take the risk, sailors said. One 32-year-old Chinese seafarer on a ship stuck in the Persian Gulf said his captain has twice refused orders from the shipowner to cross the strait.
With these tactics at its disposal, the destruction of Iran’s bigger military vessels makes relatively little difference to the Revolutionary Guard’s ability to close off the strait. Iran is thought to have thousands of mines, and they can be laid from fishing vessels or other small craft, said Long, the former British navy official.
“Iran has lost 80% to 90% of its naval capacity,” said retired Vice Admiral Robert Harward, a former deputy commander of the U.S. Central Command. “The last 10% is the hardest part.”
Write to Benoit Faucon at benoit.faucon@wsj.com