Al-Qa'ida is the most successful terrorist organisation in history. By destroying the World Trade Centre in New York in 2001 it provoked the US into launching wars damaging to itself in Afghanistan and Iraq. Al-Qa'ida aimed to destroy the status quo in the Middle East and it succeeded beyond its wildest dreams.
Its success has not been all its own doing. Ayman al-Zawahiri, al-Qa'ida's number two and chief strategist, wrote at the time of 9/11 that the aim of the group was to lure the US into an over-reaction in which it would "wage battle against the Muslims". Once the US was committed to a ground war, and no longer exercised its power primarily through local surrogates, the way would be open for Muslims to launch a jihad against America. By over-reacting President Bush, aided by Tony Blair, responded to 9/11 very much as al-Qa'ida would have wished.
In the decade since the attack on the Twin Towers "terrorist experts" and governments have frequently portrayed al-Qa'ida as a tightly organised group located in north-west Pakistan. From some secret headquarters its tentacles reach out across the world, feeding recruits, expertise and money to different battlefronts.
Al-Qa'ida has never operated like that. The closest it came to being a sort of Islamic Comintern was when it had several hundred militants based in the Tora Bora mountains of Afghanistan in 1996 to 2001. Even at that time, when it could operate more or less freely in the Afghan mountains, its numbers were so small that it would hire local tribesmen by the day to be filmed forpropaganda videos showing its men marching and training.
Many of the most important al-Qa'ida leaders from that era have since been detained or killed. But the organisation has proved so hard to eradicate because it exists primarily as a set of ideas and methods for fighting holy war. Osama bin Laden's target was primarily the US and its Western allies, though this has not always been true of local franchises. Civilians were fair game because they had chosen or tolerated evil rulers. In its fundamentalist religious beliefs al-Qa'ida is little different from Wahhabism, the puritanical and intolerant version of Sunni Islam that is dominant in Saudi Arabia.
Suicide bombing became the preferred method for al-Qa'ida to wage war. It was tactically effective because it meant that untrained but fanatical recruits willing to die could be deployed as a lethal weapon capable of killing many enemies. Moreover, the public-self sacrifice of the bomber as a demonstration of Islamic faith was an important part of a successful operation.
The CIA and other intelligence agencies were criticised after 9/11 for failing to pick up on the threat posed by al-Qa'ida early in the 1990s, but in practice it barely existed before 1996 when Bin Laden moved to Afghanistan from Sudan and, even then, he was only one among several players leading Islamic Jihadi groups.
Since 2001 al-Qa'da has continued to exist organisationally mainly as a series of local franchises. In Iraq, for instance, al-Qa'ida in Mesopotamia was led by a Jordanian, Abu Musab al-Zarqawi, who had previously opposed Osama bin Laden in Afghanistan. Expanding rapidly among the defeated Iraqi Sunni after the overthrow of Saddam Hussein in 2003, it mounted a ferocious war of suicide bombings, primarily directed against the newly dominant Shia Iraqis rather than Americans. The US itself played a role in the expansion of al-Qa'ida. In Iraq the US army spokesman in Baghdad attributed all armed attacks to al-Qa'ida regardless of who carried them out. He hoped to discredit the insurgents in the eyes of Shia Iraqis and the outside world. But within Iraq this only added to the high profile of the organisation among those hostile to the new order of things, while abroad it made it much easier for al-Qa'ida to raise money. The wave of anti-Americanism that swept the Muslim world after the invasion of Iraq also benefited the group.
One vicious aspect of al-Qa'ida activities is under-reported in the Western media: it has always killed more Shia Muslims than it did Americans. The group was sectarian before it was nationalist. The Shia were seen as heretics as worthy of death as an American or British soldier. Again and again its suicide bombers would target Shia day labourers as they waited for work in public squares in the early morning in Baghdad or massive bombs would be detonated as Shia worshippers left their mosques. Likewise in Pakistan, the Pakistan Taliban, ideologically linked to al- Qa'ida, has shown equal enthusiasm for slaughtering Shia where ever they can be targeted.
Al-Qa'ida had the advantage post 9/11 that it did not have to do much to have an impact in the US. It had entered US demonology to a degree that any action by it, however ineffectual or trivial, had a disproportionate effect: a Nigerian student, who had received training from al-Qa'ida in Yemen, failed to blow up an aircraft over Detroit using explosives hidden in his underpants; a Pakistani man living in the US was unable to detonate explosives in a car in Times Square in New York. But, as al-Qa'ida in Yemen gleefully pointed out in a statement, such failures had almost the same effect as a successful bombing in terms of the disruption and dismay caused.
No US government can afford to have another 9/11 take place without devastating retaliation from the voters. Washington had to be seen to be doing something successful to restore American confidence in its own strength. One of the reasons why Mr Bush's administration had invaded Afghanistan and Iraq, rather than devoting all efforts to hunting down Bin Laden, was that the first two options seemed easy and the third was not. Saddam Hussein was easy to puff up as a threat and eliminate in a way that was not true of the leader of al-Qa'ida.
Mr Bush set up a special cell to find Bin Laden and Zawahiri. At his morning briefings during his final months in office he would ask plaintively: "How are you getting on getting number one and number two?" In the presidential election of 2008 the Democrats made the damaging, though somewhat spurious charge, that the White House had taken its eye off the ball in the pursuit of Bin Laden in Afghanistan in order to invade Iraq.
The claim that Bin Laden was operationally ineffective also missed the point that he remained a potent symbol. This had been true since 9/11; all he had to do was to go on surviving for his survival to be a further sign that the will of the US could be frustrated. This is why Bin Laden's killing by US forces has such importance, regardless of how far he master-minded different plots or was behind more recent attacks on the US.
His demise will have some impact on al-Qa'ida itself, in so far as it exists as an organisation, but its main impact will be on American self- confidence. Of course, there will be jihadi groups who will want to restore the balance of terror by making new attacks, but none is likely to have the same impact as 9/11. The psychological effect was so great not just because so many were killed but because of the uniquely public nature of the attack: the planes crashing into the World Trade Centre and the slow crumbling of the two towers.
Will al-Qa'ida attacks be easier to carry out this year than in the past because of the fall or disruption of so many police states such as Egypt, Tunisia and Libya? The "strongmen" in the Arab world, like Hosni Mubarak or Ali Abdullah Saleh of Yemen, had post 9/11 been swift to manipulate Washington to support their despotic regimes in return for their clamping down on Islamic fundamentalists. Sometimes the repression, as in Yemen, was less effective than it looked, but in Pakistan the authorities were prepared to locate and hand over al-Qa'ida members to the US while being careful to shield the Afghan Taliban.
But the collapse of the old order in the Arab world may play against al-Qa'ida: it will no longer be the beneficiary to the extent it was in the past of the hatred felt towards local dictators allied to or tolerated by the US. Other ways of ending an intolerable political and social status quo have been demonstrated. Mr Mubarak effectively allied himself with Israel and the US during Israel's war in Lebanon in 2006 and in Gaza in 2009. This created anger among many Egyptians which benefited fundamentalist Islamic groups, but it is difficult to envisage future, more- democratic Egyptian governments being on such friendly terms with Israel. Al-Qa'ida's appeal will be diluted. But already its significance was confined mainly to the world of perceptions rather than real threats. This is why it is of such real importance that Bin Laden, the symbol of so many American fears, is dead.
Bin Laden's lieutenants
The Egyptian-born eye surgeon has been Bin Laden's second-in-command for a long time and theoretically will succeed him.
Khalid Sheikh Mohammed
The mastermind of the 9/11 attacks, he is in detention in Guantanamo Bay where he will face a military trial.
Once an important leader of the insurgency in Afghanistan, this Saudi Arabian was reportedly killed by US forces a week ago.
Sa'ad bin Abi Waqas
Killed in the same Nato air strike as Ghani, he was considered a dangerous regional commander, who specialised in training.