In January 2012, Barack Obama sent a private letter to Tehran with a stern warning: the Strait of Hormuz was a “red line” for the United States, and any Iranian attempt to close the vital shipping channel would draw a swift military response.

Obama administration officials say that in recent weeks Iran has edged closer to that line by harassing commercial shipping in the area, and the Pentagon is reacting to the threat. The flashpoint is the Strait of Hormuz, a narrow waterway along Iran’s west coast that provides access to the Persian Gulf and major oil-exporting ports in Kuwait, Iraq, and Saudi Arabia that fuel the global economy.


On Thursday, Defense Secretary Ashton Carter approved military escorts for some commercial freighters passing through the strait, which happens to be the site of the last direct military clash between Iran and the U.S. nearly 30 years ago.

Facing a combustible situation with the potential to derail President Obama’s nuclear talks, administration officials who spoke to POLITICO on condition of anonymity chose their words carefully, seeking a balance between asserting U.S. power and avoiding confrontation.

“The idea is to maintain a credible U.S. Navy deterrence to prevent escalation,” said a senior defense official. “This is nothing more than benign but prudent accompanying of already-scheduled ships. It is not designed to send a signal. It is merely a hedge against some unpredictable Iranian behavior the last few days around the Strait of Hormuz, and is not something we expect to continue for long.”

U.S. officials are still trying to interpret Iran’s actions and intentions in one of the world’s most important waterways. A series of aggressive Iranian moves since last month’s framework nuclear agreement, a triumph for Iranian moderates, have raised the question of whether hard-liners in Iran’s military are asserting themselves in response.

Earlier this week, Iran seized a container ship registered in the Marshall Islands as it passed through the strait. Iran claims that the vessel, the Maersk Tigris, is the subject of a legal dispute with its parent company. But Pentagon officials were alarmed that Iran’s navy fired a warning shot at the ship, and say Iran has taken other aggressive actions in those waters recent weeks.

History illustrates how dangerous the situation is. The last direct combat between America and Iran occurred as a result of Iranian interference with shipping in the area. Over several months in 1987 and 1988, one American missile frigate was struck by Iranian mine; several Iranian navy ships were sunk; and a U.S. guided missile cruiser mistook an Iranian passenger plane for a military jet and shot it down, killing 290 people.

As the U.S. and Iran try to seal the nuclear agreement by June 30, both sides have a strong incentive to avoid seeing shots fired. But officials and experts worry that a larger crisis could be sparked by an accident — or a trigger-happy Iranian commander — in the cramped waters. At its narrowest point, the strait is just 21 miles wide.

“It’s a very small waterway. There is a tremendous opportunity for miscalculation,” says Kenneth Katzman, a Middle East analyst at the Congressional Research Service, a government policy research office.

“I think an actual shooting confrontation would definitely cause a delay in finalizing this nuclear accord,” Katzman added.

The tensions around the Strait of Hormuz come days after another U.S. show of naval force to deter Iran. In late April American warships sailed to the waters near Yemen to challenge an Iranian flotilla that was carrying arms to that country’s Houthi rebels. The Iranian ships turned back —a victory for the U.S. But the Obama administration has been careful not to crow about the episode for fear of provoking the Iranians.

(Iran, for its part, denies any loss of face: “The news report by the foreign media that we have changed our route after the U.S. fleet’s arrival is only a media ballyhoo,” Iranian Navy commander Mostafa Tajeddini said on Tuesday, according to Iran’s Fars News Agency.)

The seriousness with which the Obama administration takes security in the strait was clear in the president’s January 2012 letter to Tehran. Obama was reacting to a threat, made by Iran’s vice president, to close the strait in retaliation for proposed western sanctions against Iranian oil exports.

“Iran has total control over the strategic waterway,” added Rear Adm. Habibollah Sayyari, Iran’s naval commander, a few days later. “Closing the Strait of Hormuz is very easy for Iranian naval forces.”

The Pentagon quickly deployed two carrier strike groups to the area in a show of strength backed by Obama’s letter (which was soon publicly disclosed). Iran never followed through on its threat, even after new oil sanctions were imposed.

Few experts expect Iran to cross Obama’s red line by shutting the strait — an act that could cause global economic mayhem. Nearly 20 percent of the world’s oil supplies pass through the waterway, according to the U.S. Energy Information Administration, which calls it “the world’s most important oil transit chokepoint.”

But a 2012 CRS report authored by Katzman warned that Iran was less likely to shut down the strait entirely than to “disrupt, threaten, harass, and otherwise create substantial instability for shipping in the Gulf.” It added that such “low-intensity” tactics could be a deterrent against “increasing economic, diplomatic, or military pressure against Iran.”

It’s not clear whether the U.S. military would respond to such an Iranian strategy. But it could cost Iran in other ways, by angering oil-consuming countries hit by a spike in crude prices—notably including China, with which Iran has sought closer economic ties.

The Strait of Hormuz | Google Maps

Amid the mounting tensions over Iran’s behavior in the region, the nuclear talks resumed on April 20 and Obama officials are optimistic about reaching a final accord with Iran, despite differing accounts in Washington and Tehran about the likely form of a final deal. On Friday, White House press secretary Josh Earnest repeated the administration’s position that bellicose Iranian behavior only strengthens the case for striking a nuclear deal that the White House says would block’s Iran’s path to a nuclear weapon indefinitely.

But the talks appear to have created a parallel competition as Iran, in the view of some analysts, demonstrates that it is not bowing to U.S. pressure while Obama maintains a firm posture to reassure Arab allies worried about a possible detente between Washington and Tehran.

Officials expect the resulting friction to last through the end of the nuclear talks this summer — and likely for years to come.

“Iran acts in a destabilizing way in a very volatile region of the world,” Earnest said. “We continue to be concerned about that.”