Blockades are designed to work slowly, with pressure accumulating over time. At the beginning of the American Civil War, for example, President Abraham Lincoln ordered a blockade of ports across the Confederacy, targeting some 3,500 miles of coastline. It had the desired effect, eventually cutting Southern cotton exports by as much as 90 percent and severely damaging the Southern economy. But it did not result in a rapid end to the war: Fighting between North and South continued for four years.
A similar story played out during the British naval blockade of Germany in World War I. Instituted almost immediately after the war began in 1914, it aimed to limit Germany’s access to essentials like food and medicine and matériel that might support the war effort. The blockade imposed severe hardship on the German people, contributing to hundreds of thousands of civilian deaths, and hampered military operations. But Germany did not immediately surrender. The war endured until the end of 1918.
That blockades often fail to quickly change an adversary’s behavior is something Mr. Trump and his advisers should know. Earlier this year, the United States started interdicting oil shipments to Cuba in an effort to force Havana to make political and economic concessions. The island is now on the brink of humanitarian collapse, but the Cuban regime has yet to yield. The U.S. blockade of Venezuela’s oil exports was similarly ineffective: Mr. Trump announced it in December 2025, part of a monthslong pressure campaign to force President Nicolás Maduro to step down. When a few weeks of blockade failed to elicit any compromise, Mr. Trump had to escalate further, seizing Mr. Maduro and his wife in a dangerous military raid.
Iran may prove even more resilient. The blockade has reduced the country’s oil revenues to a fraction of their prewar levels, but it is likely to be some time before the consequences become untenable for Iran’s regime. In the near term, Tehran will continue to receive oil revenue from shipments that left its ports weeks ago, and at least 34 tankers with links to Iran appear to have slipped through the blockade. These and any future successful exports can be sold at higher prices, which may continue to rise as the war drags on.
To prevent this, the administration has said the U.S. military will pursue any ship helping Iran, anywhere in the world, a move that is of ambiguous legality under international law. To meet the legal standard, any blockade must be deemed “effective,” meaning it is carried out with enough military power to be consistently and impartially enforced; have clearly defined geographic limits; and include provisions for humanitarian relief. The expanded U.S. blockade meets none of these requirements. It has no geographic boundaries or humanitarian provisions, and the U.S. Navy’s limited capacity to interdict container ships and tankers means it will have to choose which cargoes to intercept or focus on specific regions. It cannot, therefore, be “effective.” In the end, most Iranian oil shipments that are already at sea will almost certainly make it to their destinations.
