As investigators try to piece together the details of the plot that provoked the worst terrorist attacks in history, another important hunt has been launched. It is the intellectual quest to answer the questions: who are these people? and why do they hate the west?

The terrorism waged by Osama bin Laden's al-Qaeda, the organisation suspected of carrying out the attacks in the US, has been difficult to understand because of its enormity, its international scope and its ability to attract recruits from various Arab nationalities, as well as Arab migrants in the west. It is bloodier than other terror groups because it uses religion to gain legitimacy among recruits and convince them that suicide bombings against innocent civilians are acts of self-defence and salvation rather than atrocities.

The statements of Mr bin Laden and the analysis of experts on Islamism suggest that the al-Qaeda phenomenon is the product of the convergence of four factors that have affected the Middle East over the past two decades.

First, al-Qaeda is the result of the search for a new cause by a group of militants who were promoted by Pakistan, Saudi Arabia and the US to fight a war to liberate the Muslim land of Afghanistan from Soviet occupation in the 1980s. The Arabs recruited were groomed in "international" struggle and told by ulemas, or religious scholars, that it was their religious duty to fight the infidels. When the war was over, they joined the subsequent battles in Bosnia, Kashmir and Chechnya, fighting alongside other Muslims. Along the way, they turned against the US, which was perceived as having abandoned them after the Soviet withdrawal from Afghanistan and as having adopted a Middle East policy seen as a new form of colonisation.

The second factor in al-Qaeda's development is that the first Afghanistan jihad coincided with the rise of mainstream and largely non-violent Islamist movements across the Arab world. These were opposition groups rebelling against authoritarian regimes and social injustice. Largely promoted by middle-class and well educated politicians and scholars, they used religion to gain mass appeal. The groups promised to re-create the golden age of Islam by establishing Islamic states free of corruption and mismanagement.

In Saudi Arabia, from where several of the terrorists involved in the attacks on New York and Washington appear to have emerged, the opposition rose in reaction to the Gulf war and the royal family's acquiescence in the stationing of US troops on Saudi soil. This last point was the source of Mr bin Laden's dispute with the regime and led to his being stripped of his Saudi nationality in 1994.

At first seen as useful buffers against leftist groups, Islamist movements soon became a threat to established orders. Some governments such as Jordan and Morocco sought to co-opt them by giving them a legal but limited political role. Others, such as Algeria and Egypt, resorted to repression, which contributed to the radicalisation of a minority now suspected of working with Mr bin Laden.

As Remy Leveau, a professor at the Institut d'Etudes Politiques in Paris, says: "The violence we now see is the desperate attempt to overthrow a regime that created the [militants] but that will not allow them to take over power. Radical Islamism has failed and we are now witnessing the same desperation we see in the Basque movement."

Olivier Roy, a French expert on Islamist movements, says governments' attempts to undercut the Islamist opposition and regain religious legitimacy have backfired by producing a sometimes more radical brand of Islamism that is largely uncoupled from state policies.

Saudi Arabia is now cited as an example of this trend. The regime, whose legitimacy is based on Wahhabism, an 18th-century revivalist movement that preaches a return to a pure form of Islam, has repeatedly given in to the clerical establishment as a way of co-opting opponents. For example, the education system has increasingly promoted religious classes. Across the kingdom, religious police ensure that women follow a strict dress code and are segregated from men. The dissidents who emerged after the Gulf war and spread their messages via recorded tapes appear to have been subdued but Saudis now fear that a more radical minority that backs Mr bin Laden's international terror campaign may have emerged instead.

"International Islamic terrorism has shifted from state-sponsored actions or actions against domestic targets towards a de-territorialised, supranational and largely uprooted activism," Mr Roy wrote in 1999 in the Brown Journal of World Affairs.

This analysis may explain the third element that lies behind the September 11 terror attacks, namely that Arab migrants to the west are one of Mr bin Laden's favourite targets for recruitment. "This is part of the effects of globalisation," says Mr Roy. "These people are confronted with having to integrate in a new nation and most make the choice but a small part does not. The revolt becomes a mixture of Islamic rebellion and traditional western anarchism."

The social profile of the migrant recruit can be disenfranchised, marginalised youth or more middle-class and idealistic young Arabs, who are attracted by Mr bin Laden's focus on political issues. "There is a minority, radicalised by what they see in the Muslim world, [that] can be recruited and mobilised. Bin Laden appeals to people who feel victimisation and oppression daily - and young Arabs and Muslim ideologists," says John Esposito, professor of religion and international affairs at Georgetown University in Washington. "He [Mr bin Laden] first identifies political and economic issues, then phrases his discourse within a religious context . . . He says these are real issues; corrupt authoritarian Arab governments are the cause and it is the US and the west that support them. You live in a world where Islam is therefore under siege and you have a right and a duty to defend it."

Indeed, the regional environment is the fourth factor in the development of al-Qaeda. The core group has been aided by the climate of ihbat (the feeling of having been let down) prevalent in the Arab world. In many countries, this stems from the failure of governments to deliver on promises of economic prosperity in an oil-rich region or to follow through with pledges of political reforms.

But the disenchantment is also the product of decades of humiliation of Arab defeats against Israel, which followed a period of colonisation by the west. As a former senior Arab official says: "Arabs are brought up on ideas of unity and dignity and support for the Palestinian cause but are confronted by disunity and military defeats."

This frustration has developed into anger towards the US, which supports domestic regimes but is seen by ordinary Arabs as following a foreign policy of "double standards". Arabs criticise the US for maintaining United Nations sanctions against Iraq while ignoring Israel's obligations under UN resolutions. In the past year, the criticism has turned into anger as satellite television stations in the region have brought home daily pictures of Palestinians killed by Israeli troops in the intifada, or Palestinian uprising, against occupation. Although Mr bin Laden's main target is the US and the Saudi monarchy, his discourse has increasingly focused on Israel, winning him popularity in the Arab world.

The fact that mainstream Islamist groups, governments and opposition have forcefully spoken out against the attacks in the US may help to remove the supposed religious cover for transnational acts of terror and the justification for extremist actions. But the combination of the factors that have created and fed al-Qaeda suggest that dismantling the network will be an enormously complex task.

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