Days after the U.S. Navy began blockading the Strait of Hormuz, key questions remain unanswered about how such a large-scale operation can be sustained — and history suggests naval blockades are difficult to enforce and their results are often unpredictable at best.

The White House says it wants to choke off Iran’s main source of revenue, oil exports, by cutting the country off from global maritime trade. It’s a move aimed at increasing economic pressure on Iran after weeks of U.S. strikes have failed to persuade the country’s leaders to agree to end the war on Washington’s terms.

The U.S. Middle East command, known as CENTCOM, said on Sunday that it would intercept all vessels going to and from Iranian ports and will “not impede freedom of navigation” for ships from all other Persian Gulf ports.

Meanwhile, President Trump has made clear that stopping all shipping to and from Iran is aimed at strangling Iran’s ability to export petroleum. The administration labels the pressure tactic as a blockade — though Bryan Clark, a senior fellow at the Hudson Institute, argues it’s more of a naval quarantine, because “the U.S. is only stopping traffic that’s coming from Iran.”

Such a tactic is simply a new facet in the long-term sanctions that the U.S. has placed on Iran, says Eric Schuck, an economics professor at Linfield University in Oregon. He says the U.S. is following the classic economic pressure tactic aimed at breaking an enemy’s economy. The way to do that is finding and cutting off “something which is nonsubstitutable, something that is so essential to their economy that everything else is going to come to a halt.” In Iran’s case, that is oil.

But will the strategy work? Here are three lessons learned from the history of naval blockades.

Blockades zap resources and are hard to enforce

For much of history, naval blockades were mostly enforced through coordinated patrols, control of key routes and strategic positioning of ships. During the French Revolution and the Napoleonic wars of the early 19th century, for example, Britain imposed blockades on key French ports, which required a significant portion of the Royal Navy’s ships. And even then, some nimble French vessels — blockade runners — were still able to slip through the British screen.

Unlike the British squadrons off French ports or blockades during the 20th century, the U.S. Navy can use shipboard position beacons, satellites, drones and helicopters to locate and watch vessels coming in and out of the Strait of Hormuz, according to Steve Dunn, author of Blockade: Cruiser Warfare and the Starvation of Germany in World War One.

“Detection of vessels is much easier, with satellite, [planes and drones] and radar,” using helicopters and fast boats to send boarding parties to determine whether a ship will be allowed to pass, Dunn wrote in an email to NPR.

The Navy will likely need “six or so destroyers in rotation” to enforce the strait blockade, according to the Hudson Institute’s Clark, who is an expert in naval operations and electronic warfare. Prior to the U.S.-Iran war, an average of 138 ships passed through the strait daily. With so many vessels going through the strategic choke point, “it would be almost impossible [for the Navy] to keep up with that traffic volume,” he says.

The early months of the Ukraine war demonstrated a similar difficulty: Russia’s navy initially tried to restrict Ukrainian maritime exports from the Black Sea, using sea mines and warships to threaten commercial traffic. It resulted in a de facto partial blockade of Ukrainian grain exports, which are crucial to Ukraine’s economy. But it was “quite quickly negotiated away,” partly because Russia lacked the full military capacity needed to enforce it, according to Nicholas Mulder, a professor at Cornell University who specializes in the history of sanctions, blockades and economic warfare.

“That’s the difficult thing about blockades — you have to enforce them,” Mulder says.

The logistics of enforcing a blockade are not simple, Clark says. The blockading country’s navy must essentially pull over ships, like a traffic cop at sea. In the Arabian Sea outside the strait, the U.S. Navy “would intercept [ships] and basically get in their way and force them to turn … or take them over to a marshaling area or an anchorage in Oman,” he says.

The Navy isn’t prepared to track and stop that many ships, he says: “I don’t see the U.S. mounting a scorched-earth campaign of attacking every little vessel that tries to evade the blockade.”

They aren’t always effective

Schuck, of Linfield University, says during World War II, the Allied and Axis submarine campaigns — effectively naval blockades of shipping — provide a stark dichotomy of outcomes. The German U-boat campaign against Britain in the 1940s operated under the assumption that “if we sink everything, then it doesn’t matter. … We can cripple the British war economy,” Schuck says. However, in the end Britain was “able to make sure that the one supply line that mattered, that North Atlantic supply line,” remained open.

By contrast, the U.S. submarine campaign against Japan was “brutally effective,” targeting oil and resource flows from the Dutch East Indies to the Japanese home islands. The pressure forced Japan to shift its fleet in a way that undermined its own defense, since “they had to relocate a bunch of their fleet” just to defend their oil supply. As a result, things deteriorated on the homefront, Schuck says: By the closing months of the war, the caloric intake in Japan had dropped dramatically.

They don’t always hit their target

If history is any guide, naval blockades often have unintended consequences. “In most cases, what we’re aiming at and what we actually break are two different things,” says Schuck, who has studied the economics of naval blockades.

During World War I, the Allies imposed a naval blockade on Germany to restrict imports of strategic materials such as nitrates and phosphates used in explosives. However, these same chemicals were also critical for the production of fertilizer.

“What wound up breaking wasn’t so much the German defense industrial base — it was their agricultural sector,” Schuck says. As a result, Germany’s civilian population faced severe food shortages and widespread malnutrition in the latter years of the war.

Likewise, during the British blockade of French ports around the turn of the 19th century, French trade collapsed along with the economy.

In the case of Iran, Schuck says, its oil revenue is its lifeblood, so “there is a potential … that their food supply could be exposed from this.” But that likely depends on how long the blockade lasts or how effective it is at shutting down Iran’s commerce.