The arrest of Najibullah Zazi shatters the conventional wisdom that al-Qaeda no longer needs a sanctuary from which to plan and plot terrorist attacks. It thus strengthens President Obama's core argument that al-Qaeda must be prevented from re-establishing havens in Afghanistan and General Stanley McChrystal's request for additional U.S. forces there.

Zazi, an Afghan-born U.S. resident, has reportedly admitted to FBI agents that he was trained in bomb making at an al-Qaeda camp in Pakistan. Authorities believe that this training made Zazi's plan more likely to succeed than those of other plotters arrested in the United States in recent years-including four men who attempted to attack two Bronx synagogues last May.

It further underscores the central lesson from the September 11, 2001 attacks: that al-Qaeda is most dangerous when it has a sanctuary from which to organize attacks and provide critical training.

One of the arguments used today against a continued U.S. buildup in Afghanistan is that terrorists today no longer need training centers in order to mount attacks. Paul R. Pillar, a former senior CIA official, for instance, has argued prominently that planning for the 9/11 attacks did not take place in Afghanistan but in "apartments in Germany, hotel rooms in Spain and flight schools in the United States." But, while it is true that follow-on, tactical planning did indeed occur in those places, according to the authoritative 9/11 Commission Report, among other sources, the location and strategic genesis of the operation was indisputably in Afghanistan.

In late 1998/early 1999 the operation's mastermind, Khaled Shaikh Mohammed (KSM), in fact moved to Afghanistan at Osama bin Laden's invitation precisely for this purpose. Bin Laden subsequently took the idea forward in discussions with his military chief, Mohammed Atef, and KSM at al-Qaeda's al Matar complex near Khandahar in the spring of 1999. An "elite" training course was then organized for the four operatives originally selected to pilot the hijacked aircraft at al-Qaeda's Mes Aynak camp-also in Afghanistan. It was only later, towards the end of 1999, that Mohammed Atta and the three other Hamburg operatives entered the picture when they came to Afghanistan to undergo training at al-Qaeda camps pursuant to fulfill their desire to fight (and die) in Chechnya. It was while at these al-Qaeda camps in Afghanistan that they were recruited for the 9/11 attack. KSM confirmed all this when he was interviewed by Pakistani journalist Yosri Fouda in 2002.

And bin Laden himself, in the famous "Khandahar Tape," captured by U.S. forces in that city in November 2001 and broadcast the following month, detailed his own intimate involvement in the planning of the 9/11 attacks-from exactly where he was sitting in Khandahar.

Additional al-Qaeda operations were also planned in Afghanistan. The 1999 "Millennium Bomber," Ahmad Ressam, was trained by KSM at al-Qaeda's al Farouk facility in Afghanistan. And, the 2003 Istanbul suicide bombings were originally conceived at al-Qaeda's camp outside of Khandahar.

And, since 2004, every major terrorist attack or plot against the United States or our European allies has emanated from either al-Qaeda or the Taliban or its affiliates and associates based along the lawless border separating Afghanistan and Pakistan.

In a landmark speech that the then-director general of MI-5, Dame Eliza Manningham-Buller, gave in November 2006, she explained how upwards of thirty terrorist plots and attacks in the UK "linked back to al-Qaeda in Pakistan and through those links al-Qaeda gives guidance and training to its largely British foot soldiers here on an extensive and growing scale."

These include a 2004 planned bombing of a London nightclub and a shopping center; the July 7, 2005 suicide attacks on London transport; the follow-on, failed plot against the same target, two weeks later; and the 2006 airline bombing plot. Members of each of those cells were trained at an al-Qaeda camp in Pakistan's Malakand Agency. And the 2004 plots to attack multiple business targets in New York, Newark, Washington, DC and London were directed by senior al-Qaeda commanders based in North Waziristan.

Significantly, the terrorists involved in the 7 July 2005 attack and the failed follow-on operation on 21 July 2005, as well as the 2006 airline plot, all received the same instruction at the same al-Qaeda camp in Pakistan in the use of the homemade explosives that Zazi reportedly received, too.

Clearly, defending Afghanistan will not eradicate a terrorist network based in Pakistan. But failing to defend Afghanistan will almost certainly give al-Qaeda new momentum and greater freedom of action. Sanctuary-and in the form of something larger than an apartment's confines and in the kind of permissive environment that the border straddling Afghanistan and Pakistan currently affords-appears to be quite important to al-Qaeda and its allies. It is certainly one of the main reasons that the authorities are taking Najibullah Zazi's case as seriously as they are.

Bruce Hoffman is a professor in Georgetown University's Security Studies Program, a Senior Fellow at the U.S. Military Academy's Combating Terrorism Center, and a Contributing Editor to The National Interest.