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.

The ongoing war of words between President Trump and Pope Leo XIV is unparalleled in modern history. It’s not new for popes to speak out on political issues, historians of religion say, but Trump’s insults toward the pope are without precedent.

The direct nature of Pope Leo’s responses as well as him being the first American pope are also playing a role in how the exchange is being interpreted by the public.

The recent back and forth started with Leo’s calling for peace in response to the war in Iran, and continued with him warning of the “delusion of omnipotence” and writing that “God does not bless any conflict.”

It escalated this past weekend when Trump accused Leo of being “WEAK on Crime, and terrible for Foreign Policy,” a potential response to Catholic leaders’ calling for more humanity in the Trump administration’s immigration policies. Trump also claimed Leo was in favor of Iran having nuclear weapons. Trump continued his attacks Tuesday night with another social media post, saying, “Will someone please tell Pope Leo that Iran has killed at least 42,000 innocent, completely unarmed, protesters in the last two months.”

“I have no fear of neither the Trump administration nor of speaking out loudly about the message in the Gospel,” Leo told reporters on Monday at the start of an 11-day Africa tour.

Vice President Vance, who is Catholic, also weighed in on the controversy on Tuesday night, saying the pope should “be careful when he talks about matters of theology.”

“What we saw … is an unprecedented, unhinged attack by the president of the United States on the pope,” said Christopher White, associate director of the Initiative on Catholic Social Thought and Public Life at Georgetown University. “It was clearly meant to intimidate the pope,” but, he added, “the pope’s response shows he is undeterred by the president’s broadside and won’t be distracted from his efforts to push for peace.”

The charged nature of the exchange is new, but many popes have been known for their political critiques. Here’s a brief overview of times when modern popes spoke out on politics, and how Pope Leo is different.

Popes have had political opinions before, but the response was diplomatic

Pope Paul VI talking to President Lyndon Johnson during a special audience at the Vatican City, Rome, on Dec. 23, 1967. Pope Paul famously said: “No more war, war never again.”

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Modern popes have never shied away from voicing political opinions, sometimes running contrary to world leaders.

“When the pope speaks, it’s not that he’s taking sides. He’s really pointing out the objective moral law,” said Michele Dillon, a professor of sociology at the University of New Hampshire whose research focuses on the Catholic Church.

But prior interactions were much more diplomatic.

In 1965, Pope Paul VI was the first pope to speak before the United Nations, urging an end to the Vietnam War and famously saying, “No more war, war never again.” Paul VI pushed President Lyndon Johnson to “increase even more your noble effort” to negotiate for peace in Vietnam in 1967. Later that year, Johnson released a cordial statement after meeting the pope, saying “I deeply appreciate the full and free manner” of the pope’s opinions.

In 1979, Pope John Paul II spoke before the United Nations, focusing on human rights and peace. He advocated an end to conflicts in the Middle East, with a “just settlement of the Palestinian question” and the “territorial integrity of Lebanon.” John Paul II visited President Jimmy Carter in the White House, where they talked about the Philippines, China, Europe, South Korea, and the Middle East, according to Carter’s notes.

John Paul II, a Polish pope, was also involved in less-public political influence. He supported Polish opposition to the Soviet Union and has been credited with helping to bring down the Berlin Wall in 1989. Later, in 2003, he spoke against the U.S. invasion of Iraq and also sent representatives to Washington and Baghdad to make appeals to avoid the war. Those appeals were ignored, but he correctly predicted decades of unrest in the Middle East, according to White.

Pope John Paul II and President Jimmy Carter in October 1979.

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John Paul II also voiced opinions on social issues with presidents — disagreeing with Bill Clinton on abortion and pushing George W. Bush to reject stem cell research — but neither president escalated the situation and both remained respectful.

More recently, in 2013, Pope Francis called an impromptu vigil to plead for peace in the civil war in Syria and wrote to Russian President Vladimir Putin to oppose military intervention there. Francis responded to a chemical attack that left some 70 people dead in Syria in 2017, saying he was “horrified,” and he appealed “to the conscience of those who have political responsibility” to end the violence.

In 2015, Francis released a document saying the church accepted the scientific consensus on climate change and urged world leaders to act.

“Many of the world’s leading climate activists have said that no one has done more to shape public opinion on [climate change] than Pope Francis,” White said.

Francis was also a tireless advocate for peace in Gaza, and would call Gaza’s Church of the Holy Family nightly during the war between Hamas and Israel.

Francis also went head to head with Trump in 2016 before Trump’s first election. When Francis visited the U.S.-Mexico border, he said a person “who thinks only about building walls, wherever they may be, and not building bridges, is not Christian.” Trump called the pope’s comments “disgraceful,” but he quickly smoothed over the situation and called Francis a “wonderful guy.”

Popes have been reluctant to name names before now

Popes have historically been hesitant to name the person their criticism is directed at outright. A hotly contested example is Pope Pius XII’s decision to not directly name and denounce Adolf Hitler during World War II.

Pope Francis also faced criticism for his ambiguous references to Russia’s invasion of Ukraine.

This makes Leo’s directness all the more relevant, according to White, who is also the author of Pope Leo XIV: Inside the Conclave and the Dawn of a New Papacy. Leo referring to Trump by name, though still a rare occurrence, was a “new tact” for the papacy, he said.

“There’s just kind of a reflex on the Vatican’s behalf to want to be perceived as neutral as possible in a conflict,” he said. Leo, however, “appealed to [Trump] directly and in a sense, pointed the finger to say: ‘You started this war, you have the power to end this war.’”

The pope does not want to get involved in a political back and forth, said Dillon, the UNH professor, but his job is to preach the Catholic teachings.

“That’s the last thing any pope wants to do, because they do want to be a pope for the universal church and for all people,” Dillon said. “A pope of peace.”

The Trump administration is frequently invoking religion 

Another reason for Leo’s outspokenness may be the Trump administration’s continued religious rhetoric and imagery, experts said.

On Sunday, Trump shared an AI-generated image that depicts him as a Jesus-like figure, wearing a white robe and red sash and laying his hands on a sick, bedridden man as light appeared to radiate from his hands. The post was later deleted and Trump claimed the image was of him as a doctor.

Robert Orsi, a professor of religious studies and history at Northwestern University, said he was alarmed by the post’s connotations. He called the whole exchange with Leo “unprecedented,” and “never in U.S. history has this happened.”

On Wednesday, Trump shared a post on social media with an image of him being embraced by Jesus. Trump told reporters last week that he believes God supports the U.S. military action in Iran because “God is good and God wants to see people taken care of.” Last year, the White House posted an image of Trump as the pope.

“We have an administration, not just a president, but an administration that is speaking out in more overtly religious terms than even somebody like Jimmy Carter,” said Margaret Thompson, a professor of history and political science at Syracuse University. Carter was an evangelical Christian.

Dillon, the UNH professor, said that because of this, Leo may have felt a duty to personally reference and respond to Trump’s attacks, because he recognizes that “appeasement has a moral price.”

Jesuit priest and author James Martin told Morning Edition that “pretty much every Catholic I spoke to, from progressive Catholics to traditional Catholics, were appalled,” at Trump’s words toward the pope. “The pope is, you know, the representative of the whole church. So it’s an attack on the church.”

How Pope Leo is viewed, being an American pope

Pope Leo XIV leads a mass at the basilica of St. Augustine in Annaba on the second day of an 11-day apostolic journey to Africa, on Tuesday.

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Pope Leo is the first American pope, but he does not think of himself as just an American. “He’s the Holy Father for everyone,” said Peter Martin, a former U.S. diplomat accredited to the Holy See.

Still, that doesn’t stop people from looking at the saga from an American angle.

Dillon said the fact that the pope is American could allow him to have greater influence. Americans may have seen popes such as Francis, who were “pointed in their criticism of a great power like America,” as just “anti-America,” she said.

“But if you have a pope who was born and raised in Chicago and really a true out-and-out American criticizing in pointed terms, I actually think that carries more weight,” Dillon said.

In early April, Leo appealed to the American people “to seek ways to communicate. Perhaps with congressmen, with authorities, saying that we don’t want war, we want peace.”

“It doesn’t get more American than that,” White said. “I mean, I don’t think there’s any precedent for a pope saying, ‘call your congressman.’”