How will the U.S. blockade of Iranian ports work, and what will it achieve?

The U.S. military has initiated a blockade across the Gulf of Oman and the Arabian Sea, a move experts say is an act of war with potential for serious escalation in the midst of a two-week ceasefire.

U.S. Central Command says it is targeting all vessels travelling to and from Iran as of Monday morning, saying warships will monitor and potentially intercept vessels heading to Iranian ports, regardless of what country they’re coming from.

That means ships moving between non-Iranian ports can still pass through the Strait of Hormuz, but ships linked to Iranian trade could be stopped, searched or turned back.

This is not exactly what U.S. President Donald Trump threatened Sunday, when he said the U.S. would close the entire Strait of Hormuz.

Trump told reporters Monday that the blockade is meant to force Tehran back to negotiations after peace talks deteriorated over the weekend.

CBC News spoke with analysts about how the blockade could play out.

Why are they doing this?

The U.S. is aiming to cut off Iran’s oil exports, which are generating an estimated $200 million US in revenue a day.

Tehran has also been charging transit fees of up to $2 million US to other vessels passing through the strait, granting passage to vessels from friendly nations and blocking those from the U.S., Israel and their allies.

The backlog in the strait has caused oil prices to skyrocket in the U.S. and elsewhere.

Mark Norman, retired Royal Canadian Navy vice-admiral and fellow with the Canadian Global Affairs Institute, says the blockade is a “somewhat desperate act.” He says it’s ironic the U.S. is effectively blocking a strait it has wanted to open since the conflict started.

“This is really the only significant remaining pressure point, or point of leverage, the United States has,” he said. “This is all about money at the end of the day.”

David Carment, a professor of international affairs at Carleton University in Ottawa, says the U.S. and Israel don’t seem to have fully realized Iran’s capacity for resolve and retaliation, and says it’s “not at all clear” whether the U.S. has a larger plan going forward.

“Everything that they’ve done so far superficially appears to be lacking any long-term strategy. It seems to be incredibly ad hoc,” Carment said.

“One minute they’re negotiating, the next minute they’re posing greater and greater restraints on Iran. And meanwhile, Israel is pretty much doing what it wants on the sidelines in Lebanon, targeting diplomats and leaders within Iran, making it very difficult for a negotiating process to unfold.”

Carment says if he were to give the U.S. the benefit of the doubt, he would say the move is not as much about Iran as it is about constraining China, which is the world’s largest importer of oil through the strait.

How will the U.S. intercept ships?

According to ship tracking websites, two tankers linked to Iran — one carrying oil products and the other diesel — sailed out of the strait ahead of the Monday morning blockade deadline.

U.S. warships were spotted near the Persian Gulf earlier, as well as others further afield.

Norman says the U.S. military will have to establish patrol areas outside of the gulf to intercept vessels trying to enter the strait — a process that will involve numerous surface vessels, helicopters and long-range patrol aircrafts.

He says they will use available information that ships are required to broadcast digitally so they know who’s coming from where.

“Then the question becomes, how do you actually intercept these vessels? And I think that that’s a significant challenge,” he said.

Norman says the U.S. military could do what it did with embargoed Venezuelan ships in the Gulf of Mexico in the winter, where soldiers in some cases came down from helicopters to board and seize the vessels.

“And that gets pretty dicey, especially if any of those vessels are flagged as Chinese, for example,” he said.

Norman says the blockade itself is an act of war, and so is seizing a vessel of another flagged nation.

“From an international law of the sea perspective, we’re into a very serious escalation here.”

Will it work?

Norman says he’s not convinced the blockade is the best solution strategically, but the Americans “have painted themselves into a bit of a corner” in the war with Iran.

The move has been accompanied by inflammatory rhetoric, with Trump writing on Truth Social Monday that any Iranian vessels approaching the blockade would be “immediately eliminated.” The Iranian military and the Revolutionary Guard replied by threatening, “NO PORT in the region will be safe.”

Janice Stein, founding director of the University of Toronto’s Munk School of Global Affairs and Public Policy, told CBC News Network that blocking Iran from exporting its oil hits a major pressure point and could be successful in forcing another round of peace negotiations, though she said it’s a “dangerous” dance.

“Its economy is already in tatters. This will really increase the pressure on Iran,” Stein said.

Carment says it’s “a bit of guesswork on the part of the Americans” if they think the blockade will be the final blow that brings Iran to its knees.

He does not think the move will convince Iran to make concessions in negotiations. On the contrary, he says it will likely lead to a “re-escalation” of violence, with Iran digging its heels in further.

“Iran will now be compelled, I think, to respond and target assets that are American if they’re located in neighbouring states,” Carment said.

The U.S. and Iran agreed last week to a two-week ceasefire that is set to expire on April 22 if peace talks are not successful.