(Adds section on why Iran has cut off the strait now)

By Sarah Young and John Irish

March 13 (Reuters) - 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

U.S.-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 U.S.-Israeli war against Iran began on February

28, 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

U.S. 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 February 28 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 March 3 that the U.S. 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 U.S. and European Union-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.

(Additional reporting by Renee Maltezou and Kate Holton,

Writing by Charlie Devereux and Angus McDowall, Editing by

Timothy Heritage)