Roughly 138 ships passed through the Strait of Hormuz on an average day before the US-Iran war. To stop the ones bound to or from Iran, the Navy would need “six or so destroyers in rotation,” according to Bryan Clark, a senior fellow at the Hudson Institute and an expert in naval operations and electronic warfare.
The US began blockading the strait in April, and NPR reported days later that the questions about sustaining it were piling up. The White House says the goal is to choke off Iran’s main source of revenue, oil exports, after weeks of US strikes failed to bring Iranian leaders to the table on Washington’s terms. President Trump has said plainly that the point is to strangle Iran’s ability to export petroleum.
CENTCOM, the US Middle East command, said on Sunday that it would intercept all vessels going to and from Iranian ports and would “not impede freedom of navigation” for ships from other Persian Gulf ports.
Blockade or quarantine
Clark objects to the label. He calls it closer to a naval quarantine, since “the U.S. is only stopping traffic that’s coming from Iran.”
Eric Schuck, an economics professor at Linfield University in Oregon, sees it as the latest chapter in long-running US sanctions. The classic approach, he said, is to find “something which is nonsubstitutable, something that is so essential to their economy that everything else is going to come to a halt.” For Iran, that is oil.
What history suggests
Enforcement is the hard part. Britain’s blockades of French ports during the French Revolution and the Napoleonic wars tied up a large share of the Royal Navy, and some nimble French blockade runners still slipped through.
Technology helps this time. Steve Dunn, author of a book on the blockade of Germany in World War One, wrote to NPR that “detection of vessels is much easier, with satellite, [planes and drones] and radar.” Boarding parties arrive by helicopter and fast boat to decide whether a ship may pass. Even so, Clark said, with that many vessels, “it would be almost impossible [for the Navy] to keep up with that traffic volume.” Ships would be turned back in the Arabian Sea or taken to an anchorage in Oman. He does not expect the US to attack every small vessel that tries to evade the line.
Russia’s early effort to restrict Ukrainian grain exports from the Black Sea with mines and warships fell apart quickly. Nicholas Mulder, a Cornell professor who studies blockades, said it was “quite quickly negotiated away,” partly because Russia lacked the military capacity to enforce it. “That’s the difficult thing about blockades,” he said. “You have to enforce them.”
Outcomes in the world wars split sharply. Germany’s U-boat campaign failed to cut the North Atlantic supply line to Britain. The US submarine campaign against Japan, aimed at oil flowing from the Dutch East Indies, was in Schuck’s words “brutally effective,” and by the final months of the war caloric intake in Japan had dropped dramatically.
Blockades also tend to hit the wrong target. The Allied blockade of Germany in World War I was meant to deny nitrates and phosphates used in explosives, but the same chemicals went into fertilizer. “What wound up breaking wasn’t so much the German defense industrial base, it was their agricultural sector,” Schuck said, and civilians faced severe food shortages.
For Iran, Schuck said, oil revenue is lifeblood, so its food supply could be exposed, though that depends on how long the blockade lasts and how well it shuts down Iranian commerce.
