Why is it so easy for Iran to shut the Strait of Hormuz?

Following through on an old threat, Iran has effectively closed the Strait of Hormuz, cutting off a vital waterway that normally carries about a fifth of global oil and liquefied natural gas supplies, in retaliation for US-Israeli strikes.

As oil markets worry about a global energy crisis, the United States has said it may consider escorting vessels through the strait, which could prove very hard to secure - something Yemen's Houthis proved in disrupting Red Sea shipping last year.

About a fifth of global oil and liquefied natural gas normally passes through the strait, where traffic has dropped ‌by 97% since the US-Israeli war against Iran began on 28 February, according to United Nations data.

Why has Iran cut off the strait now?

When a commander in Iran's Islamic Revolutionary Guard Corps warned in 2011 that cutting off the strait would be "easier than drinking a glass of water", the threat to the strait had already been made many times before.

In the years since, the Guards have continued to warn they could close it, including during tensions over sanctions and Iran's nuclear programme in 2016 and 2018 and during Israeli and US strikes in June last year.

Analysts have always regarded the closure of the strait as a measure of last resort because of the long-term strategic changes it might prompt among Iran's enemies, and the potential for retaliation against its own energy sector.

The attack on Iran starting on 28 February with the killing of its supreme leader has changed that equation. Iranian officials describe the war as existential and with the Guards increasingly taking charge ⁠of strategy.

What is at stake?

The narrow passage of water between Iran and Oman that links the Gulf with the Gulf of Oman is the only sea exit for oil- and gas-producing countries such as Kuwait, Iran, Iraq, Qatar and the United Arab Emirates.

Oil prices briefly climbed to their highest level since 2022 on Monday. High oil prices could trigger another cost-of-living crisis, as happened after Russia's invasion of Ukraine in 2022, according to the United Nations.

A prolonged conflict could also cause a fertilizer shock, risking global food security. About 33% of the world's fertilisers, including sulphur and ammonia, pass through the Strait, according to analytics firm Kpler.

An extended war could fuel fears of a global economic crisis similar to those that followed the Middle East oil shocks of the 1970s.

Why is the strait so hard to secure?

Shipping lanes are just two nautical miles wide and ships must make a turn opposite Iranian islands and a mountainous coast that provides cover for Iranian forces, according to shipping broker SSY Global.

Iran's conventional navy has largely been destroyed but the Guards still have plenty of options including fast attack craft, mini submarines, mines and even jetskis packed with explosives, said Tom Sharpe, a retired Royal Navy commander.

Tehran has the capacity to produce around 10,000 drones a month, according to the Centre for Information Resilience, a non-profit research group.

Escorting three or four ships a day through the strait would be feasible in the short-term using seven or eight destroyers providing air cover, and would depend on whether the risk from mini submarines has been reduced, ‌but doing so ⁠sustainably for months would require more resources, Sharpe said.

Even if Iran's capacity to deploy ballistic missiles, drones and floating mines were destroyed, ships would still face a threat from suicide operations, said Adel Bakawan, Director of the European Institute for Middle East and North African Studies.

If the war does continue for weeks, some kind of escort will come together, said Kevin Rowlands, Editor of the RUSI Journal at the Royal United Services Institute.

"The world needs oil to flow through from the Gulf, and so there is planning ongoing to put protection measures in place," he said.

What have the US and other countries promised?

President Donald Trump said on 3 March that the US would provide protection through the strait for oil tankers, but there have already been attacks and very little is getting through.

He also said he had ordered the United States Development Finance ⁠Corporation to provide insurance and guarantees for shipping companies.

French President Emmanuel Macron said several European countries, India and other Asian states were planning a joint mission to provide protection. But he said such an operation could happen only once the conflict ends.

France is deploying about a dozen naval vessels, including its aircraft carrier strike group, to the eastern Mediterranean, Red Sea and potentially the Strait of Hormuz.

British Prime Minister Keir Starmer has spoken to the German and Italian leaders about options to provide support for commercial shipping in the strait, a spokesperson said on Tuesday.

"We're looking ⁠at a range of options there," General Caine told reporters at the Pentagon on Tuesday without providing details.

What happened in other shipping chokepoints in the region?

Yemen's Houthis, a group allied with Tehran but with a far smaller military arsenal at their disposal than Iran, managed to shut down most traffic passing through the Red Sea and the Bab al-Mandab Strait on its way to the Suez Canal for more than two years despite protection provided by US and EU-led forces.

Most shipping companies are still using a ⁠far longer route via the southern tip of Africa. Danish shipping company Maersk had said it would begin a staggered return to the Suez route from January.

An EU-led force has been more successful at countering piracy off Somalia's coast, but that has been against forces far less well equipped than Iran's Revolutionary Guards.

Are there any alternatives to using the strait?

The UAE and Saudi Arabia have sought to find ways to bypass the strait by building more oil pipelines.

But those are not currently operational and an attack on an east-west Saudi pipeline by Houthi militia in 2019 showed those alternatives were also vulnerable.