If they capture Bin Laden or his deputy will al-Qa'ida fall?

The search for the mastermind of 9/11 continues, but the network he inspires has mutated into regional groups with local recruits. Al-Qa'ida is not so much an organisation as an idea, so how can it be defeated?

America, Britain and Pakistan are stepping up their hunt for al-Qa'ida's leader, Osama bin Laden, and his deputy, Ayman al-Zawahiri, with SAS forces reported to be on their way to Afghanistan and Pakistani troops fighting fierce battles with tribesmen and foreign militants across the border.

Capturing or killing either man would be seen as a crippling blow against the movement and its campaign of global terrorism. While Bin Laden, a Saudi-born millionaire, is al-Qa'ida's facilitator and principal figurehead, his second in command, a 52-year-old Egyptian surgeon, is considered the chief strategist and ideologue. Yet even if both men were to be removed, it might not mean the end of murderous attacks such as the bombs that killed 202 commuters in Madrid 10 days ago.

Since the September 2001 "spectacular" in New York and Washington and the war that drove the al-Qa'ida leadership from its stronghold in Afghanistan, the network has decentralised. In the words of the CIA director, George Tenet, it has become "a loose collection of regional networks working autonomously ... [They] pick their own targets, they plan their own attacks".

Although al-Qa'ida's inner core has been reduced by arrests and deaths in fighting to about 1,000 militants, a quarter of its former strength, according to Dr Rohan Gunaratna, a Singapore-based expert, the tens of thousands of fighters trained in its camps in Afghanistan have dispersed to their former countries of residence. In most cases these are unstable Muslim nations such as Sudan, Yemen, Algeria and Indonesia, but a significant minority came from Western countries, including Spain - and Britain.

"Only a select few have remained as 'soldiers' in Afghanistan," said Dr Mustafa Alani, an Iraqi-born consultant to the Royal United Services Institute in London. "There is a new al-Qa'ida: the attacks in places such as Istanbul, Casablanca, Saudi Arabia and now Madrid were carried out by people recruited locally, using local resources." The explosives for the Spanish bombings were obtained in Spain, and there was an ominous absence of the electronic "chatter" that has preceded previous attacks.

"These groups don't need al-Qa'ida as a supplier of money or materials. Instead it supplies the ideology of jihad and identifies the enemy. The leadership now only gives guidelines, and these loose affiliates do the rest."

Although the network is far more loosely structured, there is still some degree of co-ordination, according to experts, with regional "field commanders" visiting groups in their countries. "This emissary might tell the leadership that a group has the capacity to stage an attack," said one source. "Later he will pass on the word that their proposal has been approved. He will be well out of the way, however, before any attack happens.

"Al-Qa'ida is prepared to expend local followers, either in suicide bombings or because they are caught by the security services after a strike, but it does its best to preserve the higher echelons."

In this new phase of "franchise terrorism", al-Qa'ida has been described as an idea rather than an organisation - "a global movement infected by al-Qa'ida's radical agenda", as Mr Tenet put it. Even if its structure has been disrupted by military action, arrests and increased security, it still acts as an inspiration to groups, from Chechnya to the Palestinian territories, that have minimal contact with the network.

But is there anything to al-Qa'ida's ideology beyond a blind hatred of America and its allies, along with some misty notion of Islam conquering the world? According to some, the movement is seeking the rebirth of a medieval Caliphate stretching from Baghdad to the Iberian peninsula, with an antique form of Islam to match, but Dr Alani disagreed.

"They may be ruthless killers who live in cloud cuckoo land, but they are very clear on their political objectives," he said. "When it came to Spain, they killed the Spanish military attaché in Iraq, then attacked Spanish forces there, killing seven officers. Last May there were the suicide bombings in Casablanca, which killed four Spaniards, and now the Madrid attacks, just before an election.

"They all put pressure on the US-Spanish alliance, and the latest one produced a government that wants to withdraw its troops from Iraq. Nobody should underestimate al-Qa'ida's ability to calculate the diplomatic and political effect of its attacks. It has a long list of victories, including the United Nations pulling out of Iraq and the withdrawal of US forces from Saudi Arabia."

Some believe that it can be counter-productive to build up Bin Laden as a bogeyman and to emphasise al-Qa'ida's ability to strike anywhere, as Tony Blair, George Bush and others, including the Metropolitan Police Commissioner, Sir John Stevens, have done. This, they say, can make the movement appear invincible, enhancing its status in the eyes of potential terrorists.

Last week London's police chief raised eyebrows when he called a terror attack on the capital "inevitable". Dr Alani said: "I can understand why he said it. He wants people to realise the need to protect themselves, but it can be misinterpreted as implying that nothing can be done. We shouldn't give up, because al-Qa'ida can be defeated."

But just as the insurgency in Iraq is still raging despite the capture of Saddam Hussein, it may take a lot more to win the "war on terror" than simply winkling Osama bin Laden out of his cave.