On July 25, Najibullah Zazi, a lanky man in his mid-twenties, walked into the Beauty Supply Warehouse in Aurora, Colorado, a suburb of Denver. The visit was captured on a store video camera. Wearing a baseball cap and pushing a shopping cart, Zazi appeared to be just another suburban guy.

Of course, not many suburban guys buy six bottles of Clairoxide hair bleach, as Zazi did on this shopping trip--or return a month later to buy a dozen bottles of "Ms. K Liquid," a peroxide-based product. Aware that these were hardly the typical purchases of a heavily bearded, dark-haired young man, Zazi--who was born in Afghanistan and spent part of his childhood in Pakistan before moving to the United States at the age of 14--kibitzed easily with the counter staff, joking that he had to buy such large quantities of hair products because he "had a lot of girlfriends."

In fact, the government believes that Zazi, a onetime coffee-cart operator on Wall Street and shuttle-van driver at the Denver airport, was planning what could have been the deadliest terrorist attack in the United States since September 11. Prior to his arrest last month, the FBI discovered pages of handwritten notes on his laptop detailing how to turn common, store-bought chemicals into bombs. If proven guilty, Zazi would be the first genuine Al Qaeda recruit discovered in the United States in the past few years.

The novel details of the case were sobering. Few Americans, after all, were expecting to be terrorized by an Al Qaeda agent wielding hair dye. But it was perhaps the least surprising fact about Zazi that was arguably the most consequential: where he is said to have trained.

In August 2008, prosecutors allege, Zazi traveled to Pakistan's tribal regions and studied explosives with Al Qaeda members. If that story sounds familiar, it should: Nearly every major jihadist plot against Western targets in the last two decades somehow leads back to Afghanistan or Pakistan. The first World Trade Center bombing in 1993 was masterminded by Ramzi Yousef, who had trained in an Al Qaeda camp on the Afghanistan-Pakistan border. Ahmed Ressam, who plotted to blow up LAX airport in 1999, was trained in Al Qaeda's Khaldan camp in Afghanistan. Key operatives in the suicide attacks on two U.S. embassies in Africa in 1998 and the USS Cole in 2000 trained in Afghanistan; so did all 19 September 11 hijackers. The leader of the 2002 Bali attack that killed more than 200 people, mostly Western tourists, was a veteran of the Afghan camps. The ringleader of the 2005 London subway bombing was trained by Al Qaeda in Pakistan. The British plotters who planned to blow up passenger planes leaving Heathrow in the summer of 2006 were taking direction from Pakistan; a July 25, 2006, e-mail from their Al Qaeda handler in that country, Rashid Rauf, urged them to "get a move on." If that attack had succeeded, as many as 1,500 would have died. The three men who, in 2007, were planning to attack Ramstein Air Base, a U.S. facility in Germany, had trained in Pakistan's tribal regions.

And yet, as President Obama weighs whether to send more troops to Afghanistan, the connection between the region and Al Qaeda has suddenly become a matter of hot dispute in Washington. We are told that September 11 was as much a product of plotting in Hamburg as in Afghanistan; that Al Qaeda and the Taliban are quite distinct groups, and that we can therefore defeat the former while tolerating the latter; that flushing jihadists out of one failing state will merely cause them to pop up in another anarchic corner of the globe; that, in the age of the Internet, denying terrorists a physical safe haven isn't all it's cracked up to be.

These arguments point toward one conclusion: The effort to secure Afghanistan is not a matter of vital U.S. interest. But those who make this case could not be more mistaken. Afghanistan and the areas of Pakistan that border it have always been the epicenter of the war on jihadist terrorism--and, at least for the foreseeable future, they will continue to be. Though it may be tempting to think otherwise, we cannot defeat Al Qaeda without securing Afghanistan.

A young Osama bin Laden first arrived in the region around 1980 to wage jihad against the Soviets; he would spend most of his adult life in Afghanistan and Pakistan. Al Qaeda leaders have, since the '80s, developed deep relationships with key Taliban commanders based along the Afghanistan-Pakistan border, such as Gulbuddin Hekmatyar and members of the Haqqani family. Bin Laden's deputy, Ayman Al Zawahiri, has even married into a local tribe.

It is true that, before September 11, some Taliban leaders opposed bin Laden's presence in Afghanistan on the grounds that it was interfering with their quest for international recognition. And it is also true that Taliban foot soldiers today are fighting for any number of reasons--ranging from cash payments, to tribal opposition to the government, to a hatred of foreigners.

But, in recent years, Taliban leaders have drawn especially close to Al Qaeda. (There are basically two branches of the Taliban--Pakistani and Afghan--but both are currently headquartered in Pakistan, and they are quite a bit more interwoven than is commonly thought.) Today, at the leadership level, the Taliban and Al Qaeda function more or less as a single entity. The signs of this are everywhere. For instance, IED attacks in Afghanistan have increased dramatically since 2004. What happened? As a Taliban member told Sami Yousafzai and Ron Moreau of Newsweek, "The Arabs taught us how to make an IED by mixing nitrate fertilizer and diesel fuel and how to pack plastic explosives and to connect them to detonators and remote-control devices like mobile phones. We learned how to do this blindfolded so we could safely plant IEDs in the dark." Another explained that "Arab and Iraqi mujahedin began visiting us, transferring the latest IED technology and suicide-bomber tactics they had learned in the Iraqi resistance."

Small numbers of Al Qaeda instructors embedded with much larger Taliban units have functioned something like U.S. Special Forces do--as trainers and force multipliers.

Meanwhile, the Taliban, like Al Qaeda, has tried to attack the West. According to Spanish prosecutors, the late and unlamented leader of the Pakistani Taliban, Baitullah Mehsud, dispatched suicide bombers on a botched mission to Barcelona in January 2008. Pakistani Taliban spokesman Maulvi Omar confirmed this in August during a videotaped interview in which he said that those bombers "were under pledge to Baitullah Mehsud." The point is not that the Taliban is going to mount a widespread campaign of terrorism in the West--it isn't--but simply that the Taliban's approach to combat has increasingly merged with Al Qaeda's.

The Taliban has borrowed more than just violent techniques from bin Laden's group. The Pakistani Taliban has an active video-propaganda operation that mimics Al Qaeda's video wing. In fact, the output of the two is often interchangeable--indicating that Taliban and Al Qaeda operations are conducted jointly. Ben Venzke of IntelCenter, a government contractor that closely monitors jihadist propaganda, reports that "a growing number of Pakistani Taliban people are showing up in Al Qaeda productions."

One of the key leaders of the Afghan Taliban as it surged in strength in 2006 was Mullah Dadullah, a thuggish but effective commander who was quite upfront about his close links to Al Qaeda. "Osama bin Laden, thank God, is alive and in good health," he told CBS in December 2006. "We are in contact with his top aides and sharing plans and operations with each other." Dadullah would later claim that bin Laden himself had supervised a Taliban suicide operation targeting Dick Cheney during his visit to Afghanistan in February 2007.

This summer, Mustafa Abu Al Yazid, one of Al Qaeda's founders and a current member of its leadership council, described his group's rapport with the Taliban during an interview with Al Jazeera in Afghanistan. "We are on a good and strong relationship with them," he explained, "and we frequently meet them." He also said that his organization continues to regard Mullah Omar as the "Commander of the Faithful"--in effect acknowledging that the Taliban leader is Al Qaeda's religious guide, a position he has enjoyed for the past decade.

The admiration is apparently mutual. For around a year now, the Saudis have been facilitating backdoor negotiations between the Afghan government and more moderate elements of the Taliban. A senior Saudi official privy to those negotiations told me that Mullah Omar has never rejected Al Qaeda.

But wouldn't the Taliban change its tune if it returned to power? Wouldn't Mullah Omar and his allies become deterrable in the same way that leaders of most other states are deterrable--and realize it is in their interest to drop Al Qaeda? This idea has been advanced by, among others, Harvard professor Stephen Walt, who wrote in August: "While it is true that Mullah Omar gave Osama bin Laden a sanctuary both before and after 9/11, it is by no means clear that they would give him free rein to attack the United States again. ... [I]f they were lucky enough to regain power, it is hard to believe they would give us a reason to come back in force."

It's impossible to know for sure. But the last time the Taliban controlled a state, it was not so interested in realpolitik; after September 11, the group made clear that it was prepared to lose everything (and it did) rather than betray bin Laden. Since then, the Taliban's leadership has grown more closely aligned with Al Qaeda's worldwide goals--not less. Today, the Taliban seems to view itself as the vanguard of a global movement that is waging God-sanctioned holy war against the infidels. Foreign policy realists want to gamble that this group, once back in power, will suddenly transform into an ultra-rational clique of Henry Kissingers. Anything could happen, I guess. But, given everything we know about the Taliban, is that really a wise wager to make?

Another common critique of the U.S. mission in Afghanistan is that there are numerous potential safe havens in the world; if Al Qaeda were facing defeat in Afghanistan, wouldn't it simply relocate to a more permissive venue? Those who raise this point are essentially talking about two things: on the one hand, the prospect of Al Qaeda moving somewhere far away like Somalia or Yemen; on the other hand, the reality that, no matter what we do to stabilize Afghanistan, its neighbor Pakistan will always be off-limits to American invasion and therefore available as a haven for Al Qaeda.

The point about Somalia and Yemen is unconvincing. Jihadists based there have shown no ability to hit targets anywhere but in their immediate neighborhoods. Many years after September 11, there is scant evidence that any senior Al Qaeda leaders have relocated to either place. For its part, Somalia is probably too anarchic, and possibly too African as well, for the largely middle-class Arab membership of Al Qaeda. In theory, of course, it's always possible that Al Qaeda could pick up and move elsewhere. But, with the exception of a few years in the 1990s, Al Qaeda has now been based in Afghanistan and Pakistan for a generation. This is the region where its leaders feel comfortable, where they have put down roots. If they didn't leave even after the United States conquered Afghanistan in late 2001, it seems unlikely that they will in the future.

The point about Pakistan serving as a safe haven is a bit more complicated. After all, what good does it do to secure Afghanistan when Al Qaeda and the Taliban are headquartered in Pakistan? Actually, plenty. "Defending Afghanistan will not eradicate a terror network in Pakistan," explains Georgetown professor Bruce Hoffman. "But failing to defend Afghanistan will almost certainly give Al Qaeda new momentum and the greater freedom of action that an expanded geographical ambit will facilitate."

There is historical precedent for Hoffman's warning. Al Qaeda was founded in Pakistan in 1988, and many of the Taliban's leaders and foot soldiers emerged out of Pakistani madrassas and refugee camps. The political vacuum in Afghanistan during the 1990s allowed these militants to expand into the country. The result, clearly, was a much stronger Al Qaeda.

The final argument against the centrality of Afghanistan has to do with the Internet. Former Army captain Andrew Exum asserted a few months ago (in a TNR Online article) that current U.S. strategy "betrays an obsession with physical space at the expense of virtual space." It is certainly true that terrorists have used the Web to communicate and, quite adeptly, for propaganda. But there is no evidence that any terrorist attack anywhere has been successfully operationalized or coordinated mainly through the Internet. I can say this with some degree of certainty because I have asked scores of counterterrorism analysts at the FBI and CIA whether a terrorist attack has ever been coordinated over the Internet--and all have drawn a blank. It is also worth recalling that the most lethal terrorist attack in history was directed from Afghanistan under the Taliban, a country with--forget the Internet--almost no phone system and little electricity.

The Web, it turns out, is of limited use for training terrorists. Take hydrogen-peroxide-based bombs, which have become something of a signature for Al Qaeda in recent years--figuring prominently in the 2005 London subway bombings, the thwarted plot to bring down seven passenger planes in 2006, the failed plan to bomb Ramstein Air Base a year later, and, now, the Zazi case. Two years ago, I spent a day at an abandoned rock quarry in the placid English county of Somerset filming a documentary about homemade bombs. My host was Dr. Sidney Alford, a leading explosives engineer with the air of the "Q" character from James Bond movies. He was conducting a demonstration to show just how effective peroxide-based bombs are--and also how difficult they are to construct correctly.

Alford concentrated some hair bleach to a high strength, combining it with an organic material--in this case, black pepper, just as the London subway bombers had done. It was a difficult process, not something that could be picked up from just sitting down in front of a computer screen. "The real skill is getting the peroxide mixture or hair bleach to the right strength," Alford explained, a process that took hours of careful preparation to ensure the proper ratios. Indeed, when amateurs try to use similar techniques, it can end messily. In 2006 in Texas City, Matthew Rugo and his roommate Curtis Jetton, both 21, were making this type of explosive when it blew up, killing Rugo, injuring Jetton, and gutting the apartment in which they were experimenting.

But the benefits of a physical safe haven go well beyond the ability to properly train bomb-makers. A flavor of life in an Al Qaeda training camp is provided by Shadi Abdallah, a Jordanian who attended the group's Al Farouq camp in Afghanistan before being arrested in Germany in 2002. Here was the account he gave to German interrogators:

After the basic training it was possible to receive special training in an area of special talent and/or interest. ... Possible areas included: tactical training in mountain, urban or desert warfare; anti-aircraft combat on rocket grenade launchers; fighting against armored targets, i.e. tanks; terrorist attacks and assassinations. ... The first subject in which we were trained was the use of firearms. The second subject included correct behavior in outside terrain: camouflage, forms of movement, deception. In the third subject they taught us how to orient ourselves outside. The fourth and last subject was the use of explosives. The arms training lasted three weeks; all other subjects involved a week of training each.

The trainees who emerge from these camps forge bonds of deep trust through communal living and shared privation. The idea that this can be replicated over the Internet is absurd. Just as the U.S. military does not train its soldiers over the Internet, nor do effective terrorist groups.

Al Qaeda's leaders are themselves keenly aware of the importance of maintaining a safe haven. The very words Al Qaeda mean "the base" in Arabic; and, as bin Laden explained in an interview with Al Jazeera in 2001, the name is not a reference to some kind of abstract foundation but, rather, to a physical spot for training: "Abu Ubaidah Al Banjshiri [an early military commander of Al Qaeda] created a military base to train the young men to fight. ... So this place was called ‘The Base,' as in a training base, and the name grew from this."

But it isn't just a safe haven that Al Qaeda wants; it is a state. As Zawahiri explained shortly after September 11 in his autobiographical Knights Under the Prophet's Banner, "Confronting the enemies of Islam, and launching jihad against them require a Muslim authority, established on a Muslim land that raises the banner of jihad and rallies the Muslims around it. Without achieving this goal our actions will mean nothing." No wonder Al Qaeda remains so committed to Afghanistan--and so deeply invested in helping the Taliban succeed.

Of course, the centrality of Afghanistan to the war on terrorism is separate from the matter of whether we can actually secure the country. But, while Afghanistan will not be transformed into a stable country easily or quickly, we should take heart from the fact that the Afghan people want us to try. Most polls since the fall of the Taliban have found that a majority of Afghans hold a favorable view of the international presence in their country. Nationwide surveys conducted earlier this year showed that 62 percent of Afghans view the United States favorably and 63 percent support the U.S. military. By contrast, the Taliban's favorable ratings are consistently below 10 percent. In counterinsurgency theory, the center of gravity of the conflict is the population. The fact that the Afghan people remain both overwhelmingly opposed to the Taliban and also quite supportive of us--even after all of the mistakes we have made over the past eight years--should tell us that securing the country is not an impossible task.

The flawed election of Hamid Karzai and the pervasive corruption of his government, to be sure, raise a serious question connected to another key aspect of counterinsurgency doctrine: Is there a legitimate government to support? But Afghans, who have not had much functional government in their lives, actually want something much simpler than that: They want security. That is why many of them at first embraced the Taliban in the mid-1990s--because it at least delivered this paramount public good. In the short term, it is probably impossible to significantly reform the Afghan government. But security is something that international forces can provide. And, once there is security, perhaps over the long term there will be openings for political change as well.

For years, we in the United States have known about Afghanistan only in the context of war. And so, it has become difficult for us to believe that things there could ever be any other way. But not so long ago, during the mid-twentieth century, Afghanistan was a country at peace with itself and its neighbors.

It can be a peaceful nation again. And, if America is to keep Al Qaeda at bay, it must be.