(Paul Cruickshank is a Fellow at the NYU Center on Law and Security).

Although there have undoubtedly been more plots launched by home-grown cells in the West since 9/11 than by Al Qaeda, the evidence suggests that the most dangerous plots, such as the July 2005 London bombings and the 2006 airlines plot, have all been directed by Al Qaeda.

Although Matt Levitt is right to point out that the genesis of the Madrid bombings remains somewhat murky, there is evidence that the attacks were more closely linked to Al Qaeda than was initially thought. The bombing operation itself was carried out by a Spanish based cell of the Moroccan Islamic Combatant group (GICM), an Al Qaeda affiliate, whose leadership built up close personal ties with Al Qaeda in the 1990s in Afghanistan. In the weeks after the Madrid attacks, Belgian police rounded up a GICM cell based in the Flemish town of Maaseik that provided key logistical support for the attacks. One of the leaders of that cell was Lahoussine el Haski, who Belgian authorities believe helped coordinate the launch of Al Qaeda's terrorist campaign in Saudi Arabia in May 2003.

Plots sponsored by terrorist networks such as Al Qaeda, the Moroccan Combatant Group or other substantially sized terrorist groups tend to have more chance of success because cell members can draw on significant financial, technical, and logistical support.

At least up till now, autonomous "self generated" cells have proven to be more amateurish than Al Qaeda and easier for security services to round up.

Home-grown cells tend not to have the sort of terrorist tradecraft or bomb-making expertise that Al Qaeda operatives develop during training in terrorist camps. Nor the same heightened sense of mission and sense of Islamic obligation that Al Qaeda can inculcate in its recruits in the mountains of North-western Pakistan.

Terrorist training often makes a large difference because it's much more difficult than is generally realized to make a bomb by downloading instructions from the Internet. To my knowledge there have not yet been any 'successful' bombings launched by jihadist-terrorists in the West, in which the plotters relied exclusively on the internet to learn how to make a bomb.

There is always the chance, though, that untrained home-grown cells could get lucky. In Europe, because it has a large Muslim community with significant numbers alienated from mainstream society, a rapid sequence of even small attacks could lead to an anti-Muslim backlash and a vicious cycle of recrimination that could have significant consequences for social cohesion and public safety. Marc Sageman's warnings about the growth of home-grown terrorism are therefore being listened to carefully in Europe.

As far as the United States is concerned, Bruce Hoffman for my money, is absolutely correct to stress that Al Qaeda Central (i.e. Al Qaeda operatives trained in Pakistan) poses the real danger. A few small 'home-grown' attacks are not going to tear apart the social fabric in the United States.

I'm going to take a different view to Matt Levitt on the importance of Bin Laden. Since he founded Al Qaeda 20 years ago, Bin Laden has been its inspirational force and key unifying figure. The latter role has been especially important. Al Qaeda's ability to launch operations of global scope depends on being able to unite different Jihadist factions and groups, but such factions have had a tendency to quarell with eachother - to disastrous effect - about even minor theological and ideological differences.

Bin Laden's capacity to inspire operatives was demonstrated by the martyrdom tape recorded by the alleged mastermind of the 2006 airlines plot Abdullah Ahmed Ali:

Sheikh Osama warned you many times to leave our lands or you will be destroyed and now the time has come for you to be destroyed and you have nothing but to expect floods of martyr operations.

At a time when Al Qaeda is coming under increasing criticism for its tactics from a range of other Jihadist actors, Bin Laden is arguably the one figure who has the charisma and appeal in Jihadist circles to push back. Ayman Zawahiri, for example, has nothing close to Bin Laden's appeal. Without Bin Laden, Al Qaeda may still have its camps, but there will be a leadership vacuum at the top a time when the organization is entering troubled waters.

-- Paul Cruickshank

This week long terrorism salon will continue to be hosted by The Washington Note and UN Dispatch.