The United States has ramped up its forces in the Middle East, even as President Trump has pledged to wind down the war against Iran soon. In recent days, more than 5,000 Marines, paratroopers and special forces have arrived in the region, raising the prospect of a ground invasion.

Mr. Trump threatened last week to invade Iran’s main oil export hub, Kharg Island, and “obliterate” its facilities if Iran does not allow shipping to resume through the Strait of Hormuz. Iran has essentially closed it off since coming under attack by the United States and Israel a month ago.

To get to Kharg Island, U.S. amphibious forces — which experts say would most likely be a part of any ground operation — would have to make their way some 500 miles into the Persian Gulf. “That would be very risky,” said Mark F. Cancian, a senior adviser at the Center for Strategic and International Studies and a retired Marine Corps colonel. “That’s why I think that opening the strait might be first.”

U.S. officials say the president is also weighing whether to seize islands that lie in and near the strait in a bid to open the waterway, which in ordinary times carries a major portion of the world’s oil and gas.

With military outposts on multiple islands, as well as the shoreline, Iran can quickly blanket the narrow shipping lanes with drones, antiship missiles and fast-attack speedboats.

Given Iran’s firepower, the United States would have to capture the entire cluster of islands to try to open the strait, said Farzin Nadimi, a senior fellow at the Washington Institute, including Qeshm, Larak, Abu Musa and the Tunb islands. “They need to take all of them,” he said.

The United States has deployed 2,000 paratroopers, as well as special operations forces, to the region. If they landed on the islands, they could dismantle tunnel networks and underground missile bases that are “inaccessible even to bunker buster bombs,” Dr. Nadimi said.

Commanders would need to decide whether to destroy the facilities and withdraw, or hold the islands longer term to help secure the strait. They might also offer the United States leverage in negotiations with Iran.

But staying longer would require heavily equipped Marines and air defenses to protect them from Iranian drones, missiles and rockets from the shoreline. That would be a “high-risk, high-casualty undertaking,” Dr. Nadimi said.

A ground operation alone would not guarantee vessel traffic would return to the strait in meaningful numbers.

“You have to reassure the mariners, you have to reassure shipping companies and insurance companies that it’s safe enough to sail through,” said Andreas Krieg, a senior lecturer in the School of Security Studies at King’s College London.

Even with a seizure of one or multiple islands in or near the strait, Kharg Island would have significant strategic value for Mr. Trump.

Ninety percent of Iranian oil is exported from the island, and heavy U.S. strikes there in March did not halt oil shipments. Satellite images showed tankers continued to fill at the island’s export terminals in the days after the strikes.

Experts say that a U.S. seizure of Kharg Island would put a significant strain on the Iranian economy. “It is the Achilles’ heel of the regime’s oil export infrastructure,” said Behnam Ben Taleblu, senior director of the Iran Program at the Foundation for Defenses of Democracies.

But any attempt to land U.S. troops on the island would also be dangerous because of its hilly terrain and military presence. The flammable oil being stored there poses its own threats.

Experts warn that Iran could take a scorched-earth policy, destroying the oil installations on the island instead of allowing U.S. forces to capture them. And for the Trump administration, there are political risks.

If the Marines took an island but failed to force an Iranian surrender, any eventual withdrawal could look like a defeat. And if they tried to hold any islands, Iran would most likely try to turn it into a war of attrition.

“What they want is maximum casualties on the Americans, because that creates the sort of images that will change public opinion in the United States,” Mr. Krieg said.