Within days of the war’s start, Iran’s military exerted control over the strait, menacing commercial tankers with boats, missiles and drones. Shipping ground to a halt. Energy prices soared. And Mr. Trump was backed into a strategic corner.

Three months later, Iran’s control of the strait has become its most powerful weapon, a source of huge leverage in negotiations with Mr. Trump over the country’s nuclear program.

A president used to bending opponents to his will has struggled to conceal his exasperation. In an April social media post, Mr. Trump profanely demanded that the “crazy bastards” leading Iran open the strait, “or you’ll be living in Hell.” Iran’s military mocked Mr. Trump’s threat as a sign of helplessness.

But Iran’s response has been neither crazy nor surprising, say numerous former U.S. officials who spent hours war-gaming Tehran’s likely response to a major U.S. attack.

For years, the U.S. government has conducted war games dealing with potential conflicts with Iran, including ones at the Pentagon attended by dozens of military officials and policymakers. Over and over, participants say, they concluded that Iran would respond to a major American attack by closing the strait of Hormuz.

“Every single time, the first thing we focused on was the strait — without exception,” said Dennis B. Ross, a senior national security official in the Obama White House. “We assumed that if you go to war with Iran, this was their counterpoint.”

Mr. Trump has been aware of that risk since at least his first term as president. John Bolton, who served as national security adviser to Mr. Trump in his first term, recalled trying in vain to persuade the president to launch a regime-change war against Tehran. The Strait of Hormuz was always central to those discussions, Mr. Bolton said.

“It is impossible to believe that Trump was surprised by the closing of the strait,” Mr. Bolton said. The real question, he added, was why the Trump administration seemed so unprepared for that outcome.

Olivia Wales, a White House spokeswoman, said that thanks to detailed planning, “the entire administration was prepared for any action taken by the Iranian regime.”

“President Trump knew that Iran would try to stop the freedom of navigation and free flow of energy, and he took action to destroy numerous mines and over 40 mine-laying vessels,” she added.

But a look back at the run-up to the war makes clear that Mr. Trump both underestimated Iran’s ability to shut down the strait and overestimated America’s ability to reopen it if necessary. While the White House has not disclosed the details of its planning, experts and former officials said the publicly available evidence suggests several likely culprits.

One simple explanation is that Mr. Trump may have expected Iran’s government to fall before it could close the strait. Some Trump officials also believed — mistakenly — that Iran could not close the waterway without sacrificing its own oil exports and would not commit “economic suicide,” as one called it.

Mr. Trump and his top officials also appeared to believe that if Iran did try to seize the strait, American allies would help U.S. forces regain control of the waterway. That, too, was a miscalculation.

Iran’s tactics may have surprised the U.S. military. Pentagon planning focused on the assumption that Iran would heavily mine the waterway. Iran has instead relied mainly on shore-based missiles and its relatively new arsenal of cheap drones to attack and menace ships.

Mr. Trump inherited a geographic problem that has worried American strategists since the early Cold War, when they feared that the Soviet Union might try to control the channel through which roughly 20 percent of global oil supplies now flow.

Over the past two decades, amid rising tensions over its nuclear program, Iran has often harassed traffic in the strait and even threatened to close down the waterway.

After one round of such threats, in late 2011, Mr. Obama sent a secret message to Iran’s supreme leader, Ayatollah Ali Khamenei, warning that interference with the strait was a U.S. “red line” that would draw a severe military response. Iran backed down. The lesson, Mr. Ross said, was that Iran would not risk its leadership’s survival for the strait.

But Mr. Trump’s attack at the end of February reversed that calculus, launching airstrikes that killed Mr. Khamenei and other Iranian officials, and calling for the fall of Iran’s government.

“We were going for regime change,” said Kenneth M. Pollack, a former C.I.A. intelligence analyst and vice president for policy at the Middle East Institute. “That’s the key — that’s why the Iranians closed the strait.”

Mr. Trump may have expected — or at least hoped for — a quick change in government that would preclude action by Iran in the strait. Prime Minister Benjamin Netanyahu of Israel assured Mr. Trump that Iran’s government could be toppled. And Mr. Trump was still riding the high of a January commando raid that captured President Nicolás Maduro of Venezuela.

At least some Trump officials doubted that Iran would even want to close the strait, presuming that such a move would bring an end to Tehran’s lucrative oil revenues. Iran has long evaded heavy U.S. sanctions by illicitly exporting oil through the strait.

“It’s economic suicide for them if they do it,” Secretary of State Marco Rubio told Fox Business last June. “And we retain options to deal with that.”

But Mr. Rubio’s “economic suicide” scenario also hung on another mistaken assumption: that Iran could not halt most traffic through the strait without giving up its own oil exports.

At a Senate hearing on Tuesday, angry Democratic senators pressed Mr. Rubio to assure them that Mr. Trump would not make concessions to Iran simply for restoring the strait to its prewar status.

Most analysts have long assumed that Iran would make the waterway impassable by laying dozens or even hundreds of mines in its waters. That would make the strait too dangerous even for its own tankers to navigate.

The fact that Iran did not try to close the strait after a wave of U.S. airstrikes, known as Operation Midnight Hammer, against its key nuclear facilities a year ago may have supported Mr. Rubio’s view.

But Iran sidestepped that problem by using fewer mines than expected — perhaps thanks to U.S. attacks on its mining boats — and relying on missiles and drones to terrorize shipping. Ships carrying Iranian oil, which were not subject to missile or drone attacks, continued to traverse the strait for weeks, until Mr. Trump imposed a counter-blockade on Iranian shipping traffic in April.

Iran war games in the Biden administration did not imagine that drones would play such a major role in closing the strait, according to one former senior official.

“They didn’t focus enough on drones,” Mr. Bolton said of Trump officials.

During a Senate Armed Services Committee hearing last June, lawmakers questioned Adm. Brad Cooper, who would become the head of U.S. Central Command, about the Iranian threat to the strait and the military’s ability to counter it. Admiral Cooper referred to “mine warfare” and U.S. minesweeping abilities, but did not mention drones.

Acknowledging that such a scenario would be “complex,” he indicated that the military could handle it in a matter of “weeks and months.”

One former Pentagon official said the U.S. Navy was all too aware of the threat that drones could pose to shipping, thanks to attacks on commerce in the Red Sea by Yemen’s Iran-backed Houthi militants, which began in late 2023. But the U.S. military has struggled to develop effective anti-drone defenses.

Trump officials also appeared to hope that American allies would come to their aid if Iran choked off the strait. “I think the whole world would come against them if they did that,” Mr. Rubio predicted in an interview with CBS’s “Face the Nation” last June.

On March 3, Mr. Trump declared on social media that American warships would begin escorting tankers through the waterway “as soon as possible.” By mid-March, his energy secretary, Chris Wright, assured a CNBC interviewer that military escorts were “quite likely” by the end of that month, and Treasury Secretary Scott Bessent said escorts would happen “as soon as it is militarily possible.”

On March 10, Mr. Wright even posted — then quickly deleted — a claim on social media that the U.S. Navy had escorted a tanker through the strait. Officials blamed an unnamed staff member for the false claim.

But no U.S. allies beyond the immediate region have volunteered to join what many consider Mr. Trump’s reckless war of choice.

A coalition of nations led by Britain and France says it is willing to help police the strait, but not until the United States and Iran reach a formal agreement to reopen it.

Mr. Trump announced a limited “humanitarian” operation in May, called Project Freedom, to rescue tankers stranded in the strait. But he abandoned it after just one day, after Saudi Arabia protested that it risked a dangerous escalation. (In recent weeks, the United States has quietly guided around 70 commercial ships through the strait, though far from enough to have an impact on global markets and supply chains.)

A unilateral U.S. military operation to open the strait would involve major risk for a president already facing anger from supporters who believed his past vows to avoid messy Middle Eastern wars.

Mr. Pollack, who has run or participated in several simulated U.S.-Iran conflicts, said that such an operation would require deploying at least one Army division on Iran’s coast to hunt down its full arsenal of boats, mines, missiles and drones. “You have to go almost door to door on the northern shore of the strait to do this,” he said.

“It’s always been a very difficult problem,” he added. “I have not been surprised by anything the Iranians did.”