Frank Gardner The BBC's security correspondent

If the machine is not broken, why fix it? That appears to have been the strategy behind the latest al-Qaeda plot just thwarted by Moroccan and Western intelligence.

This week it was announced that Saudi members of al-Qaeda planned to load a dinghy with explosives and ram it into a Nato warship - probably US or British - in the Straits of Gibraltar.

If al-Qaeda was planning to repeat the tactics used in one of its most successful operations against the West, then it could well be considering a repeat of last year's hijackings

Ring a bell?

In October 2000 al-Qaeda pulled off exactly such an operation in Yemen's Aden harbour - its suicide bombers blew a hole in the side of a $1bn warship, the USS Cole, killing 17 American sailors.

The Americans never saw it coming.

The ship did not sink as al-Qaeda had hoped, but US pride was spectacularly dented.

The point had been made - al-Qaeda, which is dedicated to driving Western forces out of the Arabian Peninsular, could make the US military feel extremely unwelcome in Yemen.

Surveillance success

This time the Pentagon, and Whitehall, have been more fortunate. A stunningly successful intelligence operation by Moroccan, US, British and other agents managed to nip the plan in the bud.

They had the Saudis and their Moroccan wives under observation for some time, then pounced as the Saudis were preparing to leave the country.

But here is the important part - if suicide bombing a Western warship was indeed al-Qaeda's plan, then its leadership has clearly decided that what worked in Yemen could work just as well off Gibraltar.

In other words, why change a winning formula?

Possible repeat?

The implications are dramatic. If al-Qaeda was planning to repeat the tactics used in one of its most successful operations against the West, then it could well be considering a repeat of last year's hijackings, or of the East African embassy bombings of 1998.

This year much of the speculation about al-Qaeda's next move has focussed on whether it could have got its hands on a weapon of mass destruction.

The talk is of potential nuclear, chemical, biological or radiological attacks, but to score a hit against the West, and show its supporters that it is still in business, al-Qaeda may not need to use such a sophisticated device.

The attacks of 11 September are believed to have cost less than $500,000. Plans for a second major attack are thought to have been already laid.

Destruction and disruption

So now, as the giant US security and intelligence machine grapples with its biggest overhaul in history, it will need to look both forwards and backwards.

It will have to cope with the possibility of al-Qaeda deploying a weapon of mass destruction - or disruption - and it will have to be on its toes for a much more conventional attack that could still take a devastating toll.

As in the old clich� about Margaret Thatcher and the IRA, the government will have to be lucky every time. Its enemy only has to get lucky once.