https://www.wsj.com/world/middle-east/how-the-u-s-navy-is-blockading-the-strait-of-hormuz-to-choke-off-irans-ports-0a3f9784?mod=WSJ_home_mediumtopper_pos_2

American military forces began blockading all traffic attempting to enter and leave Iranian ports on Monday, setting up a risky new showdown after peace talks in Pakistan collapsed this past weekend.

With enough warships, a blockade could intimidate many tankers attempting to move oil to and from Iran. But U.S. forces would also need to be ready to board and seize hostile ships that try to break the blockade. Former and current officials said it can be done, but it is a complex operation that requires significant U.S. military resources.

Here’s what to know:

How will the blockade work?

Washington kicked off the operation Monday morning by positioning more than 15 U.S. warships, according to a senior U.S. official.

Placing resources and warships near Iran’s coast could make U.S. assets susceptible to attack, officials said, so the U.S. will likely try to intercept or quarantine commercial vessels on either side of the Strait of Hormuz to prevent them from entering or leaving Iranian ports.

Any warship can approach a tanker suspected of violating the blockade, but if a ship doesn’t comply with a request to board, specially trained Marines and special-operations forces, such as Navy SEALs, would be needed to conduct a contested boarding.

The U.S. Navy has significant firepower in the region that could support the blockade, including an aircraft carrier, multiple guided-missile destroyers, an amphibious assault ship and several other warships, according to Navy and Central Command officials. Most of these ships have the ability to launch helicopters that support boarding operations, and some are capable of marshaling commercial vessels to specific areas to hold them in place.

The U.S. could also fly boarding teams to ships from land, including from nations that border the Persian Gulf.

Iran’s “shadow fleet,” a network of oil tankers that secretly export oil in defiance of international sanctions, will likely sit out the fight, but Iranian-flagged ships are more likely to attempt to transit the waterway, said retired Vice Adm. John Miller, a former commander of U.S. Naval Forces Central Command. Those vessels may be guarded by a detachment of Islamic Revolutionary Guard Corps forces, he said.

The U.S. can track the movements of commercial vessels with surveillance tools, open-source data and military assets as they leave the Persian Gulf and exit the Strait of Hormuz, according to retired Navy Vice Adm. Kevin Donegan, who led U.S. Navy forces in the Middle East. Gulf nations that have drones and surveillance platforms could also assist the effort.

If the U.S. begins taking control of tankers, it may need a captain and crew to drive the vessels if the current crew doesn’t comply. The U.S. would also need a place to park the tankers, former officials said.

Why is the U.S. blockading the Strait of Hormuz?

President Trump’s goal is to put additional economic pressure on Iran by cutting off one of its remaining revenue streams, according to the senior U.S. official.

The move came almost immediately after cease-fire talks collapsed. Trump is demanding Iran fully open the Strait of Hormuz free of tolls for passage; end all uranium enrichment; hand over its highly enriched uranium; accept a broader security framework that includes regional allies; and end its support of militant proxy groups in the region. The negotiations collapsed after Iran refused to give up its nuclear program, which U.S. and Israeli strikes haven’t fully dismantled.

“We can’t let a country blackmail or extort the world, because that’s what they’re doing,” Trump said at the White House on Monday. “Iran is doing absolutely no business, and we’re going to keep it that way.”

The hope is that squeezing the regime economically will compel it to resume negotiations for a political solution.

“Without a source of income, I think it becomes very difficult for the regime to continue to stay in power,” said Miller.

How did the idea for a blockade come about?

A military plan for blockading the Strait of Hormuz has been sitting on the shelves at U.S. Central Command headquarters for years. The U.S. Navy has successfully conducted other various blockades in the past, including halting shipments of Iranian weapons to Yemen’s Houthi rebels, according to retired Gen. Frank McKenzie, a former Centcom commander.

Military planners refined the concept as an option for the operation against Iran, and Adm. Brad Cooper, the current Centcom chief, presented it to Defense Secretary Pete Hegseth months ago, according to the senior U.S. official. President Trump then decided to move forward with a blockade in response to failed cease-fire talks this past weekend, the person said.

Why didn’t the U.S. do this sooner?

The operation is both resource-intensive and potentially risky for American servicemembers. The narrow strait is exposed to Iran’s coastlines, and any operations through the waterway are vulnerable to Iranian mines, drones and attack boats.

Before the war, U.S. officials decided against a blockade because they believed Iran would likely respond by seizing tankers carrying oil from U.S. partners or by mining the strait, moves that would drive up oil prices sharply.

This kind of retaliatory move remains a concern.

What if Iran retaliates?

Iran’s military has been significantly degraded, but it still retains substantial firepower with which it can threaten any ships trying to block the strait. Iran is believed to still possess thousands of ballistic missiles, and dozens of fast-attack boats belonging to the Islamic Revolutionary Guard Corps could be used to attack nearby vessels.

If Iran fires on the American ships enforcing the blockade, or U.S. and IRGC forces get into a firefight aboard a tanker, that could escalate tensions. Trump could potentially decide to renew large-scale or more limited strikes against Iran, a move that he’s recently discussed with top aides.

The senior U.S. official said the likelihood that Iran will shoot at the American ships involved in the blockade once they start seizing tankers is low, because their defensive capability has been so decimated. But Iran has threatened to attack neighboring ports in the Persian Gulf and the Gulf of Oman in response to the blockade.

This explanatory article may be periodically updated.

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