I met Usama Bin Laden on a frigid night in March 1997, deep in the barren mountains of eastern Afghanistan, a year after he first declared war on America. The mysterious Saudi multi-millionaire, a tall figure with an aristocratic demeanor, walked with the help of a cane. His calls for attacks on U.S. targets were delivered in a mild manner, belying the rage of his words.

A key to his holy war against America may be found in his childhood. His father, Mohamed Bin Laden, emigrated in 1930 from Yemen to Saudi Arabia, where he founded a construction company and became one of the richest men in Arabia. Mohamed combined business acumen with a deep religious faith, traits that he passed on to some of his 50 or so children. The family had the singular honour of renovating and maintaining Islam's holiest sites, Mecca and Medina - contracts that are, coincidentally, some of the most lucrative in the Middle East. Bin Laden was 10 when his father died in a plane crash. He has said that his life today is a continuation of the religious devotion of his father.

For many Muslims around the world, the Soviet invasion of Afghanistan was a life-transforming event - the godless communists had invaded a sovereign Muslim nation. Within weeks of the invasion, 22-year-old Bin Laden was travelling to Afghanistan's neighbour, Pakistan, to support the holy war against the Russians.

Bin Laden was already an expert in demolition from the family construction business. When he made his first trips into Afghanistan, he took hundreds of tons of construction machinery, bulldozers, loaders, dump trucks and equipment for building trenches that he put at the disposal of the Afghan guerrillas. The machinery was used to build roads, dig tunnels into the mountains for shelter and construct rudimentary hospitals.

In 1984 Bin Laden set up a guesthouse in Peshawar, Pakistan, for Muslims drawn to the jihad. It was called Bait ul Ansar, or "House of the Helpers", and was a waystation for volunteers heading for training with one of the Afghan factions. Later Bin Laden formed his own military unit and set about recruiting Muslims worldwide.

The recruits came to be known as the "Afghan Arabs", though they came from all over the Muslim world. Some were high school students whose visits were not much more than the equivalent of a summer camp. Others spent years fighting the communists. Nobody knows their exact number, but most estimates suggest the low tens of thousands. They received some sort of military training and were indoctrinated in the most extreme interpretation of jihad.

In 1986 Bin Laden founded his first camp inside Afghanistan. It was near the village of Jaji, a few miles from the border. With a force of about 50 Arabs, Bin Laden fought off a lengthy siege by Soviet forces, his baptism of fire. Arab journalists based in Peshawar wrote daily dispatches extolling his exploits, which were published in the Middle East and brought him a flood of new recruits.

"What we benefited from most was that the glory and myth of the superpower was destroyed not only in my mind, but also in all Muslims," he told me in 1997.

With the withdrawal of the Soviet Union from Afghanistan in 1989, Bin Laden turned his attention to other jihads, founding Al-Qaeda, or "the Base". Al-Qaeda's main target is the United States.

Since the mid-1980s Bin Laden had advised friends to boycott American goods because of US support for Israel and Middle Eastern regimes, such as Egypt, which he regards as "un-Islamic". His distaste for America mutated into hatred by 1990, when Iraq invaded Kuwait and American troops were dispatched to Saudi Arabia. Armed "infidels" of both sexes were trespassing on sacred Arabian soil. For Bin Laden, this defied the dying words of Mohammed: "Let there be no two religions in Arabia."

Bin Laden's war against the United States started with small operations. Bombs went off outside two hotels in Yemen housing US servicemen in 1992, killing an Australian tourist. He is implicated in the deaths of 18 US servicemen in Somalia in 1993 and the bombing of the World Trade Center in New York that same year. In 1995 men influenced by Bin Laden's writings bombed a military facility in Saudi Arabia, killing five American soldiers.

On August 7, 1998, exactly eight years after American troops landed in Saudi Arabia, the American embassies in Kenya and Tanzania were blown up and more than 200 people died. Two years later, the bombing in Yemen of the USS Cole killed 17 American sailors.

Bin Laden's foot soldiers are put through rigorous training for this war. L'Hossaine Kherchtou, a Moroccan who had been a student of catering in France in the late 1980s, travelled to Afghanistan to attend an Al-Qaeda training camp in 1991. On his first night he was awakened at 1am by gunfire and was told: "Don't think you are going to sleep in this camp."

He was trained on the American M-16 rifle, the Russian AK-47 rifle and PK sub-machinegun, the Israeli Uzi submachinegun and anti-aircraft guns. He also took classes on grenades and was taught about the use of explosives such as C3, C4 and dynamite, anti-personnel mines, anti-truck mines and butterfly mines, which children sometimes mistake as a kind of toy. After graduating from his camp, Kherchtou moved to Peshawar, where he was inducted into Al-Qaeda.

Bin Laden is not involved in day-to day dealings with his followers. He sets the general policies of Al-Qaeda, which are then relayed down a loose chain of command to lower members of the group, many of whom have had little or no contact with Bin Laden himself.

At the most, Bin Laden's core following comprises several hundred operatives. His closest aides are former economists, engineers and doctors. A second tier of several thousand militants is believed to operate beneath them. These men are to be found in at least 40 countries, including much of Africa and Europe, all of the Arab world, India, Pakistan, the Philippines, Russia and both North and South America.

An important clue about how Tuesday's attacks on America were pulled off lies in the story of Ali Mohammed, a former Egyptian army officer and member of Islamic Jihad who was jailed for his part in the bombing of America's embassies in Africa.

Mohammed settled in America as a "sleeper" years before the bombings. He became a US citizen and rose to the rank of sergeant in the US army's special forces at the same time as he was secretly giving weapons training to Islamic militants in New Jersey. The FBI stumbled on him only as part of their investigation into the 1993 World Trade Center bombing.

In 1997, the media information officer for Bin Laden's Kenyan cell, who would also play a key role in the bombing of the embassy in Nairobi, noted in his computer that the cell's mission was to attack Americans, but "we, the east Africa cell members, do not want to know about the operations plan since we are just implementers".

The suicide bombers in Nairobi were never given instructions by Bin Laden directly. As he told ABC News after the blast: ". . . it is our job to instigate. By the grace of God we did that and certain people responded to this instigation."

Another of Bin Laden's sleepers was Ahmed Ressam, a member of Algeria's Armed Islamic Group, who planned to bomb Los Angeles airport some time around New Year's Day, 2000.

Ressam left Algeria for work in France during the early 1990s. In 1994 he moved to Montreal, Canada, where he survived on welfare payments, stealing suitcases from tourists and engineering credit card frauds. Despite his life of petty crime, he was a regular worshipper at mosques. In 1998 he travelled to Al-Qaeda's Khaldan camp in Afghanistan, where he spent several months training with a group of Arabs, Germans, Swedes, French, Turks and Chechens.

Ressam received the usual tutorials on weapons and explosives, graduating to specialised classes on how to make electronic circuitry for bombs. He was also taught how to attack airports, railroads and military installations.

At the camp, a cell of fellow Algerians agreed that, after their training, they would meet up in Canada, an agreeable base because of the relative ease with which they could enter the United States. There they would rob banks and use the money for terrorist operations against American targets. Other militants at the camp were planning similar attacks in Europe, the Gulf and Israel.

In February 1999, Ressam returned to Montreal with $12,000 from Al-Qaeda, a bomb-making manual and hexamine, a booster material for explosives. He spent much of November and early December 1999 in Vancouver, holed up in a hotel, constructing four bomb-timing devices from Casio watches. He then loaded his bomb-making materials in a hired car for the trip to the United States. Luckily, he never made it past the border. He was arrested by a sharp-eyed Customs agent who noticed that he was sweating despite the chilly December weather. His arrest averted what would have been the catastrophic bombing of Los Angeles's busiest airport during the Christmas season.

Now it seems that there were perhaps dozens of Ressams living in the United States, biding their time for years, polishing their English - and in some cases, learning how to fly commercial jets.

Since the attacks on the American embassies in east Africa, the west has stepped up its intelligence operations against Bin Laden. It has used at least one mole at a medium level inside Al-Qaeda, recruited Bin Laden's former chief financial aide, Sayed Tayib al-Madani, and questioned around 20 suspected bin Laden associates. Western intelligence agencies believe that al-Qaeda organisation was responsible for several incidents in the months leading up to last week's atrocities:

FBI officers investigating last year's attack on the USS Cole in the port of Aden were withdrawn from Yemen in June after intelligence reports suggested they were to be the target of attacks by terrorists linked to bin Laden. A video has been circulating in the Middle East showing Bin Laden wearing a traditional Yemeni dagger and reciting a poem in which he hails the sailors' murders. Spanish police in June arrested Mohamed Bensakhira, a 34-year-old Algerian believed to have been planning to bomb the European parliament in Strasbourg under orders from Bin Laden. His arrest, in Alicante, followed a raid in Frankfurt last December in which three other people were arrested. Both Bensakhira, who escaped the raid, and those arrested in Germany, are believed to have been members of the same terror group.

Two weeks ago Indian police accused Bin Laden of plotting to blow up the American embassy in New Delhi. This month British, Israeli and American security services were alerted after a passenger believed to be one of Bin Laden's bag-men was injured when a commercial flight crash landed at Malaga airport. The man was discharged from hospital and officers lost him.