(Recasts with Trump demand for allies to help reopen the strait)

By Sarah Young and John Irish

March 16 (Reuters) - U.S. President Donald Trump has

demanded help from allies to reopen the Strait of Hormuz, which

usually carries about a fifth of global energy supplies, but

even if he can assemble a major coalition it could prove very

hard to end Iran's blockade.

Iran lies along one side of the narrow strait and has

responded to the U.S.-Israeli attack from February 28 by using

drones, missiles and mines to make the vital waterway unsafe for

the colossal oil and gas tankers that slowly traverse it each

day.

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 again 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 the hardline Guards have

increasingly taken 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 jet skis 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 DOES TRUMP WANT AND WHAT ARE U.S. ALLIES DOING?

Trump said on Sunday he expected many countries would send

warships and demanded that they do so, adding that his

administration was in touch with seven countries about helping.

That step came a week after he ordered the U.S.

International Development Finance Corporation to provide

insurance and guarantees for shipping companies.

British Prime Minister Keir Starmer has discussed the need to

reopen the strait with Trump and had previously said London was

working with allies on a range of options.

EU foreign ministers are on Monday to discuss bolstering a

small naval mission focused on protecting Red Sea shipping from

Yemen's Houthis, but are not expected to discuss expanding the

mission to the Strait of Hormuz.

French President Emmanuel Macron said before Trump's demand that

several European and Asian countries were planning a joint

mission to provide protection, but only after the conflict ends.

Germany has said it is sceptical about even bolstering the Red

Sea mission, saying it had not proven very effective.

Japan and Australia both said on Monday they were not

planning to send naval vessels to help escort ships through the

strait.

WHAT HAPPENED IN OTHER SHIPPING CHOKEPOINTS IN THE REGION?

Yemen's Houthis, a group allied with Iran but with a far smaller

military arsenal at their disposal, shut down most traffic

passing through the Red Sea for more than two years despite U.S.

and EU naval efforts.

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 and Keith Weir)