Iranian threats hobbled shipping through the Strait of Hormuz during the war, moving what little remained north into its own waters.

Before war Feb. 18–27

After Iranian retaliation April 3–12

Now the U.S. has mounted a blockade of its own, redrawing the board once again.

After the U.S. blockade

U.S. ships enforcing blockade in Gulf of Oman

U.S. ships enforcing blockade in Gulf of Oman

The American blockade aims to upend a dynamic that had become the new normal in the Strait of Hormuz since the United States and Israel launched the war in late February: Iran allowed ships carrying its own cargo to pass through the strait, even as it attacked commercial vessels and effectively halted shipping from almost everybody else.

Though Iran was getting much-needed revenue from the Iranian-linked oil tankers it allowed to pass, the U.S. allowed them to keep transiting the strait. The goal was to temper the sharp increases in oil prices tied to the war.

On Monday, the United States imposed its own naval blockade, intent on ending Iran’s dominance of the waterway and cutting off its oil income by blocking all traffic to and from its ports.

More than 12 American military vessels were stationed in international waters in the Gulf of Oman, beyond the strait, a U.S. official said on Tuesday. And the military is likely monitoring the region from a distance, using radar, patrol aircraft and drones, said Jennifer Parker, a former naval officer now at the University of Western Australia’s Defense and Security Institute.

Since the U.S. blockade took effect, no ships linked to Iran have been spotted leaving the region, according to the vessel‑tracking company Kpler.

Some ships appeared to have slowed or stopped. And at least two that had links to Iran, and are the target of U.S. sanctions, appeared to have turned around back toward the Persian Gulf as of Wednesday. One of the ships that reversed course, the Rich Starry, a Chinese tanker, was spotted traveling eastward through the strait on Tuesday toward open water before making a U-turn.

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​​Some ships without links to Iran did move through the strait on Monday and Tuesday, according to U.S. Central Command and companies like Kpler. The vessels stayed close to the Omani coast, keeping a distance from possible sea mines in the middle of the waterway.

A precise accounting of how many vessels are crossing the strait is difficult, because vessels can hide or falsify information about their location, according to maritime intelligence experts.

How Iran, then the U.S., changed shipping

Vessel traffic in the strait slowed almost immediately after the United States and Israel attacked Iran on Feb. 28, from around 130 ships per day to just a handful.

But even ships without ties to Iran may remain hesitant to attempt a passage. Amid fears that Iran may attack commercial vessels in response to the U.S. blockade, many shipping companies have been unwilling to risk the crossing. That might not change in the absence of a longer-term deal between the United States and Iran.

Around 900 ships have been bottled up in the Persian Gulf over the course of the war, according to a New York Times analysis of Kpler data.

The standoff between the United States and Iran has spread concern that the vessels will be there even longer, giving the Iranians the upper hand, said Andreas Krieg, a senior lecturer in the School of Security Studies at King’s College London.

“We’re getting to a place where everyone is very desperate, so the Iranians are trying to milk it as long as they can,” Mr. Krieg said. “I think we’ll have months and months of disruption around the Strait of Hormuz.”