Al-Qaeda's ability to regroup and plan future terrorist acts is dependent upon its key personnel, now on the run from Afghanistan, securing ties with up to 100,000 militants scattered around the world, and activating affiliated command cells dispersed in over 50 countries.

Since September 11, several attacks have been secretly thwarted. More publicly, a plot involving militants linked to al-Qaeda in Malaysia, Indonesia, the Philippines and Singapore, intended to reduce the US, UK, Australian and Israeli embassies in Singapore to rubble, was thwarted by chance only days before the bombers were due to strike.

Signs that al-Qaeda's global network has retained an ability to function have also emerged since Richard Reid - the "Shoebomber" - almost succeeded in detonating explosives hidden his shoe on board an American Airlines flight from Paris to Miami. Reid relied on a network of Europe-based operatives, as well as email contact with operatives in Pakistan, investigators are now convinced.

In an effort to destroy this network, security forces have arrested 500 alleged al-Qaeda activists worldwide. A total of 147 countries have frozen $104m of assests belonging to 168 groups or individuals with alleged links to terrorist financing.

But as the probability grows that much of the senior al-Qaeda leadership - including Osama bin Laden himself - has escaped the offensive in Afghanistan, the US has now identified a key bin Laden lieutenant, a Palestinian know as Abu Zubyadah, as likely to play a major role in al-Qaeda's future operations.

Bin Laden's current location remains a mystery, though the likelihood that he may be in the Tribal Areas of north-west Pakistan is widely accepted. Ayman al-Zawahiri, his deputy, has similarly disappeared. Only one senior al-Qaeda official - Anas al-Libi - is now in the hands of US forces, while doubts remain as to how senior or well-informed are those now being held at the US Camp X-Ray base at Guantanamo Bay on Cuba.

Despite the vast breadth of the campaign to crush al-Qaeda, however, the question on the mind of most security services is not whether the network will strike again, but when.