But a framework deal that President Trump signed with Iran last month helped bring that effort to a fiery end because of its language giving Iran official power in the strait and its vagueness in important phrases.

Mr. Trump celebrated the agreement, reached on June 14, as the reopening of the strait. “Ships of the World, start your engines,” he wrote on social media. “Let the oil flow!”

But critics say it actually formalized a reality that Iranian officials have made clear throughout the war: They now control the strait.

Iran’s missile and drone attacks on commercial ships in the strait essentially shut it down soon after the United States and Israel started the war. Then weeks after the United States and Iran entered a tentative and informal cease-fire in early April, some tankers began taking a southern route through the strait, farther from Iran’s coast.

Now, by striking last week in that area, Iran is trying to force ships to travel through its territorial waters on the strait’s northern side, where Tehran can try to justify charging tolls or fees.

Iranian units attacked three ships on Tuesday along the southern route, the U.S. military said. Mr. Trump responded by ordering airstrikes in Iran. The tensions escalated in an announcement this weekend by Iran’s Navy that it had fired on another vessel in the strait and was closing the waterway “until the end of U.S. interference in the region.”

U.S. Central Command said it hit about 140 Iranian military targets in response, for a total of 310 American strikes over the last week.

With Mr. Trump warning that he thinks the June agreement is “over,” global energy prices are surging again, along with fears of a return to all-out war. Before the war, a fifth of the world’s oil and liquefied natural gas passed through the strait from producers in the Middle East.

The latest crisis is an all-too-predictable result of the June agreement, former American officials and analysts say.

Facing anger over high gas prices and spiking inflation, Mr. Trump was eager to reopen the strait and relieve pressure on the global economy. Among other things, he agreed to end a U.S. military blockade of Iranian ports and allow Tehran to resume oil sales for 60 days in return for reopening the strait.

The June agreement, called a memorandum of understanding, also prompted additional negotiations aimed at reaching a broader and more enduring peace plan.

While many U.S. and foreign officials welcomed the cease-fire, critics warned that the agreement was dangerously vague, particularly language in its fifth paragraph saying that Iran would “make arrangements using its best efforts for the safe passage of commercial vessels” through the Strait of Hormuz.

“No one should be surprised that Iran views that as explicitly giving them an enduring role controlling passage through Hormuz,” said Michael Ratney, a retired career diplomat who was the most recent U.S. ambassador to Saudi Arabia.

“Iran’s control obviously gives them powerful leverage,” he said, “and they appear to be willing to risk a resumption of conflict, perhaps even a collapse of the cease-fire, to maintain that leverage.”

At a news conference on June 18, Vice President JD Vance insisted that Mr. Trump’s demands about the strait would be enshrined in a future deal.

“We have all the cards,” he said.

The competition for control of the strait poses a dilemma for shipping companies: Should they pass through the Strait of Hormuz via the southern corridor closer to Oman and risk being attacked by Iran? Or should they take the northern Iranian corridor, paying high fees and reinforcing Tehran’s claims of authority?

A Fraught Document

For almost 60 years, commercial ships sailed through the Strait of Hormuz along a route established by the United Nations.

Iran’s government supported the creation of the route in 1968 and did not try to control it even though it passed through Iranian territorial waters.

The leaders who took power in Iran during the 1979 Islamic Revolution said they were not bound by that U.N. agreement, though over the years the government has only occasionally challenged shipping in the strait.

That changed after U.S. and Israeli forces attacked Iran on Feb. 28.

Iran’s military quickly began striking commercial ships and laying mines, grinding traffic to a halt. Only those willing to pay large sums to Iran were granted safe passage along its coast.

Critics say that Mr. Trump conceded this new status quo in the June agreement with Tehran. At the insistence of Iranian negotiators, that 14-point document acknowledges Iran’s power in the strait.

It prohibits the charging of tolls or fees, but only for 60 days while negotiations toward another agreement continues. (Mr. Trump has said the United States could try to charge tolls.) The memorandum also does not include an ironclad guarantee that ships can safely sail any portion of the strait.

Iranian officials and diplomatic experts say the final line formally ceded to Iran a central role in managing the strait: “The Islamic Republic of Iran will conduct dialogue with the Sultanate of Oman to define the future administration and maritime services in the Strait of Hormuz, in discussion with other Persian Gulf littoral states, in line with the applicable international law and the sovereign rights of coastal states of the Strait of Hormuz.”

At the time, Mr. Trump praised the agreement as a return to free navigation through the strait. But Iranian officials were soon citing it as grounds for dictating where ships should sail — namely, along a route near Iran’s coastline.

Dennis Ross, a former longtime Middle East negotiator for presidents of both parties, said Iran’s view of the agreement was clear.

“You were opening the strait — but only on the condition that Iran was completely in control and that any other routes are not acceptable,” Mr. Ross said.

Hussein Ibish, a scholar at the Arab Gulf States Institute, a research and advocacy group, said “all of international law goes in one direction, and the M.O.U. goes in the other direction.”

Asked for comment, the White House referred to a Friday phone briefing that it had arranged for reporters on the condition that the U.S. officials speaking not be named. An American official involved in the negotiations said that Iran knew during the talks that ships were using a route near the Oman coast and had even fired drones at some of them. That meant when Iran agreed to make “best efforts” for safe passage, it would not launch attacks there, the official said.

Naval Guidance

Iranian negotiators knew they had leverage over the Americans as they discussed the proposed agreement in the early summer.

On May 4, the American military started an operation called Project Freedom to begin opening the strait by escorting stranded commercial ships.

Mr. Trump abandoned the effort in less than 48 hours after the crown prince of Saudi Arabia, fearing Iranian retaliation, refused to allow the Americans to use Saudi airspace for the venture.

The Pentagon then tried a more subtle effort mainly involving radio guidance.

Since early May, U.S. forces have given route guidance along Oman to more than 800 commercial vessels carrying 400 million barrels of crude oil, said Capt. Tim Hawkins, a spokesman for U.S. Central Command, which oversees the American military in the Middle East.

The ships followed a route designated by the International Maritime Organization, a London-based arm of the United Nations that regulates global shipping. The organization established the route in consultation with Oman to try to evacuate some 600 long-stranded vessels.

An informal cease-fire turned into a formal one with the signing of the June agreement. In the seven days starting June 20, nearly 400 ships transited the strait, according to Kpler, a maritime data firm. That was the highest number in a one-week period since the war began.

But on Thursday, after Iran’s attacks, just 22 ships went through the strait.

More than a dozen U.S. Navy warships, including two aircraft carriers, and scores of carrier- and land-based attack and surveillance planes are still operating in the general area of the Arabian Sea. The U.S. military is also conducting mine-detection missions in the strait using autonomous sea craft.

“U.S. forces have held Iran accountable for its unwarranted aggression toward commercial shipping while still facilitating passage through the strait,” Captain Hawkins said.

However, he added, there is “no guarantee” that American military guidance will protect commercial ships transiting the strait.

The Iranian Passage

During the height of the war, some shipping operators chose to sail closer to Iran — and rely on the Iranian military’s guarantee of safe passage. Iran told them they would have to pay up to $2 million per ship.

Iran has said that any ships transiting the strait must follow that route, and get permission to do so from the Persian Gulf Strait Authority, a body Tehran created in May.

Until it paused the practice under the terms of the June cease-fire, Iran insisted that the fees were for safety and environmental services. Some experts call that a contrived attempt by Iran to appear compliant with the United Nations Convention on the Law of the Sea, which allows such fees under specific conditions.

In fact, they say, Iran is actually establishing de facto tolls, which the convention outlaws. Iran signed but never ratified the convention, so it says the terms do not apply to it. The United States also has never ratified the convention.

The United States and some other countries rejected Iran’s demand that ships use that northern route, and in response, the U.S. military established the southern route in May along Oman’s coast.

After signing the agreement last month with Iran, Mr. Trump declared that the route was “totally safe, secure, and pristine.”

But as Iran and the United States vie for leverage, mainly using their militaries, risks to shipping companies could increase, said Dan Alamariu, the chief geopolitical strategist at Alpine Macro, an investment research firm.

Iran has suffered economic pain but may be willing to endure more. Mr. Trump last week reinstated a U.S. ban on Iranian oil sales that he temporarily waived last month. But he has yet to reimpose a naval blockade of Iran’s ports.

Mr. Alamariu said, “the question is which cracks first: the Iranian economy or the global economy?”

Reporting was contributed by David E. Sanger from Washington, Farnaz Fassihi and Peter Eavis from New York, and Jenny Gross from London.