The mass attack by Yemen's Houthi rebels against vessels in the Red Sea on 9 January was their biggest yet and it signals two things.

Firstly, the Iranian-backed Houthis, who control their country's Red Sea coastline, are not backing down in the face of international pressure.

Secondly, they clearly have a powerful arsenal of missiles and drones and they are not afraid to launch them at Western warships.

In the event, all 21 drones and missiles were shot down by a combination of US Navy F/A-18 fighter jets and missiles launched from US and British warships.

But this is expensive, very expensive, and the Houthis know it.

A basic Houthi explosive drone costs just under £16,000, while a Sea Viper missile of the sort carried by HMS Diamond costs in excess of £1m. Then there are all the fuel and other costs of maintaining the multinational US-led flotilla of warships under the banner of Operation Prosperity Guardian.

"It's clearly not practical in the long term, either logistically or economically," says Royal Navy Rear Adm (retd) John Gower, "to combat a mixture of medium and low-cost missiles and drones with very expensive high-end interceptor missiles."

The risks, too, are mounting.

Older US Navy officers will not have forgotten the humiliation of al-Qaeda's attack on the USS Cole in Aden harbour in 2000. Using a simple speedboat packed with explosives, al-Qaeda suicide bombers rammed it into the side of that billion-dollar warship, crippling the ship and killing 17 US sailors.

Onboard close-range defences have improved since then, but Western warships are not impregnable.

Iran, which........

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