More than a decade ago, in the wake of the 9/11 attacks, many asked a very simple question: what is al-Qaeda?
The answers were manifold. Many were erroneous, wrongly describing the group founded by Osama bin Laden and a small number of associates in Pakistan in 1988 as a tightly-knit, hierarchical organisation with sleeper cells and networks established across the globe. These interpretations underestimated the ideological element of al-Qaeda and gave the group undue prominence in the varied landscape of contemporary Sunni Muslim militant activism.
Ten years on, analysts now largely agree on what al-Qaeda is and how the group - or rather the phenomenon - has evolved over recent years. Though differences of opinion still exist, often linked to a broader political sensibility on the left or right, a consensus has formed.
For many, the phenomenon of Al-Qaeda can usefully be divided into various elements: the hardcore leadership, the various affiliated groups, the ideology and those attracted by it. The question is now which of these elements is dominant.
However this is to misunderstand the fifth phase - which we are in now.
The historical survey above is important exactly because it is not merely historical. All the various elements found in the four previous phases exist today, complimenting and reinforcing one another. There are the lone wolves and those simply inspired by the ideology and acting independently. There are the affiliates and the networks, some new, some old, some strong, some weak, but all evolving fast and often dangerously. There is still the vanguard - Ayman al'Zawahiri, the veteran Egyptian militant who is the current leader of al-Qaeda, is still alive - and new leaders committed to the same strategy of spectacular violence as bin Laden was could still emerge.
And finally there is the general overall situation, which has many elements reminiscent of the late 1980s and early 1990s. There is the chaotic and increasingly fragmented world of the various Sunni militant groups. There is the massive historical events: the collapse of communism in the late 80s and the Arab Spring today. There is the sheer parochialism of the earlier period.
What is lacking, happily, is the complacency and ignorance vis-à-vis Islamic militancy that characterised the Western response to the phenomenon in the run up to 2001, particularly when it comes to the "homegrown" threat in NATO countries. Back then it became clear that though knowledge is power, the powerful are not always knowledgeable. At great cost, that has been put right.
The threat of homegrown terrorism, if still present, is diminished as a result. It does not pose an existential threat to our societies, if it ever did. This at the very least is a very significant achievement.
Over the 24 years since the formation of al Qaida, four phases can be distinguished. Each helps us in different ways to understand the nature of the threat from radical Sunni Islamic extremism today, particularly when it comes to so-called homegrown terrorism
Looking back over the 24 years since the formation of the group in the last years of the war in Afghanistan against the Soviet occupiers, four phases can be distinguished. Each helps us in different ways to understand the nature of the threat from radical Sunni Islamic extremism today, particularly when it comes to so-called homegrown terrorism.
The first phase can be termed the pre-al-Qaeda phase. For too long after the 1998 attacks on US embassies in Tanzania and Kenya and then 9/11 itself brought the group to global attention, al-Qaeda was seen as synonymous with the broader world of Islamic militancy. A historical phenomenon with roots in successive waves of religious revivalism and resistance to Western influences going back centuries was reduced to the work of one man and his group.
But in fact al-Qaeda emerged from a whole series of scruffy, chaotic local insurgencies, all with deep roots in a range of local and global factors, all with long histories, all deeply parochial, with some pragmatic and short-term alliances between organisations but no over-arching structural unity. Indeed it was this very disunity that prompted bin Laden to envisage his group.
This has important lessons for today.
One of the key elements here for understanding the dynamics of militancy within NATO countries today is the links between volunteers in western countries and far-off conflicts. From the UK, for example, thousands of young men of Pakistani-origin headed off to Kashmir in the 1990s to fight in the insurgency there against Indian security forces. A smaller number left France to fight in Algeria - and the violence travelled the other way with Algerians committing violent attacks on French soil. In France too, the situation in their former colony contributed to local "homegrown" radicalisation.
Nearly 20 years later, we see young American or British men from Somali origin heading to east Africa, or Germans of Turkish origin heading to Pakistan to link up with Turkic organisations such as the Islamic Movement of Uzbekistan or its various offshoots. Historical connections to source countries of immigrants have remained important throughout the last three decades. There seems no reason why they should not do so in the future.
By 2003 or 2004, new security arrangements made it impossible to insert large groups of foreign operatives in target countries. The vanguard was replaced by the "network of networks"
The second phase, from the early 1990s to roughly 1998, could be called the "vanguard" phase. One understanding of al-Qaeda - al-Qaeda al'Sulbah - was formulated by the key ideologue and strategist Abdullah Azzam in the 1980s and can be translated as a revolutionary elite of hardened, committed, visionary operatives who - through their actions - would spark radical change.
This is how bin Laden and his close associates saw themselves during the 1990s. Their operations followed this template. The bombings of the east African embassies in 1998 was executed by small groups of highly-trained and highly-motivated specialists who were flown in for the purpose and backed up by local recruits. The 9/11 attacks too followed this model. So did other strikes in the immediate aftermath of that operation. This is still the model that groups such as al-Qaeda in the Arabian Peninsular and the remaining al-Qaeda senior leadership in Pakistan's tribal regions would like to follow if they could.
The third phase was not long coming. By 2003 or 2004, new security arrangements made it impossible to insert large groups of foreign operatives in target countries. The vanguard was replaced by the "network of networks". Neither independent, nor entirely controlled by al-Qaeda leaders, this dynamic, rapidly evolving matrix of cells, individuals and affiliated groups, held together by personal associations, tribal and family links and shared experiences, posed the greatest threat through the middle years of the decade.
The attacks in Bali, Madrid and London repeatedly showed how this hybrid form of organisation posed the greatest danger. One where "homegrown" volunteers provided the manpower and the local knowledge, while the central core bring the necessary logistical expertise, strategic direction and, crucially, legitimacy. That danger still exists, as failed plots in the US and elsewhere have shown. Since 2007 or 2008, it is the "ideology" of al-Qaeda that has taken over as dominant.
For several years, there have been an increasing number of so-called "lone wolf" attackers, inspired by but in no way linked to al-Qaeda.
This is particularly the case in the US, though there are many examples in other NATO countries. In the UK, in May 2010, a young woman with no previous history of violence stabbed a member of parliament who had supported the war in Iraq. In March 2012, a young French Muslim with no obvious links to established groups or even networks went on a killing spree directed at soldiers of north African origin and Jewish targets. Helped by the propaganda work of people like the late Anwar al'Awlaqi, the ideology of al-Qaeda -al-Qaeda-ism- has mutated into a well-established sub-culture of jihad with its own language, dress code, internet sites and so on. This remains attractive to a large number of young people, even if only a very few of them go on to become involved in actual violence.
Significant though the death of bin Laden himself was, analysts largely agree that the al-Qaeda core leadership had been getting weaker for some time before the death of their leader. One British security official told me last year that it was entirely conceivable that the entire era of Bin Laden and al-Qaeda's apparent dominance of contemporary Islamic militancy was not only drawing to a close but would soon be seen as "an aberration".