Credibility?
Are you sure it wasn't because 2000 people were killed by attacks set off by an organization operating out of Afghanistan?
And the need to destroy that organization so it doesn't happen again?
The fact that the organization was based in Afghanistan is why we attacked Afghanistan and not, say, Tanzania. However, the reasoning for attacking Afghanistan is a lot more complex than simply "they attacked us, therefore we attack them."
You see, even the complete destruction of Al-Qaeda in Afghanistan/Pakistan wouldn't do a damned thing about the wider problem of terrorism. Fighting terrorism is a great deal more complicated than taking out the single organization that orchestrated the attacks of 9/11. On a certain level, it is not only difficult but
impossible to completely prevent terrorist attacks on the United States or any other country, for that matter. No country can be completely secure from terrorism. That's a hard truth that it would do well to recall in
any argument about the subject. Most Americans haven't really internalized this, but given that it takes only a few people to conduct an extremely dramatic attack on the United States, it's basically pointless arguing about it. The entire American security establishment understands this in private, and it gets very frustrating when leaders and law enforcement officials promise that no further attacks will occur. They
hope no further attacks will occur. They will take measures to
try to prevent further attacks from occurring. But they cannot by any means
guarantee that such attacks will not happen again.
What all this has to do with attacking Al-Qaeda is that the folks holed up in Afghanistan are not necessary to international terrorism. AQ is as much a symbol as it is an organizing network, and ever since we hit Afghanistan, its organizing capabilities have been severely hampered. On the other hand, people still try international terrorism under the Al-Qaeda banner. Remember the underpants bomber? The organization he claimed to come from, Al-Qaeda in the Arabian Peninsula, actually has very little to do with the Al-Qaeda leadership in Pakistan. It started calling itself "Al-Qaeda in the Arabian Peninsula," and the AQ leadership in Afghanistan said, "Sure, whatever floats your boat." Al-Qaeda doesn't need to even exist for another 9/11 to happen. It helps having an heir to a construction empire bankrolling such dramatic operations, but you can still get some pretty dramatic results with fewer resources if you have enough people--and given that 9/11 required only 19 hijackers and one organizer, "enough" is not a lot. In other words, even if you completely destroyed Al-Qaeda, killed Bin Ladin, etc., etc., etc., you would
still have the problem of international terrorism. And now that Al-Qaeda amounts to little more than 500 men running scared in the mountains of Pakistan (CIA estimate, with British, French, Russian, and Pakistani intelligence all concurring), we really don't need to worry about Al-Qaeda anymore.
On top of that, invading Afghanistan to get Al-Qaeda is, to use an old Chinese phrase, like lighting a furnace to burn a hair. Al-Qaeda never numbered more than 2 or 3,000 men under arms and 20,000 theoretical and material supporters (these are also CIA numbers). Most of those supporters were outside Afghanistan, sending money, arms, and the like but not actively involved in fighting. Afghanistan is a mountainous country of 28 million people. Any military operation against Al-Qaeda by the United States would necessarily had to have been overkill if that was its sole purpose.
Of course, this was because the Taliban were sheltering Al-Qaeda. However, we frankly didn't have much of a beef with the Taliban until 9/11. Please recall that we had negotiated with the Taliban leader the previous year in order to help Unocal build that pipeline. Throughout the 90s, we never really gave a damn about Afghanistan other than that. We did continue to recognize the Northern Alliance as the legitimate government of Afghanistan, but that was mostly because everyone else did; Saudi Arabia and Pakistan were exceptions, and served as good intermediaries.
So let's consider the United States' strategic choice after 9/11. You know that the organization that attacked you consists of a couple thousand crazies in holed up in Afghanistan. Setting aside issues of calming your frightened populace and maintaining your credibility as a global power, what's the calculus? On one hand, the crazies will be difficult to find and harder to kill or capture, and they enjoy the protection of the Afghan government. That means that if you want to fight them, you'll have to fight the Taliban. On the other hand, you have no desire to fight the Taliban: besides the fact that the Soviets proved that fighting a war in Afghanistan is a nigh-impossible proposition, you've actually got a good thing going with the Taliban. They recently agreed to let you build a pipeline across their territory (or whatever), and an invasion would delay construction. They're close to the Saudis and Pakistanis and enemies of the Iranians. In other words, the Taliban aren't a government you'd be particularly keen to fight, for reasons of cost and geopolitics.
Or in short: Without factoring the fact that both the frightened American people and the international community were expecting the US to do
something about Al-Qaeda, there would be no reason for the US to do
anything in Afghanistan.
However, the people were scared: they wanted the government to do something to prove that they could keep them safe. And the international community, while hardly scared, was deeply interested in what the United States would do next: Would it sit around and dither, or would it act decisively? If the former, it would be a troubling sign: it would mean that the United States had either lost the will or the capability to act on a global scale. That would make it a superpower no more. Its allies would rush to find new friends, and its enemies and rivals would be only too glad to oblige. If the latter, the United States could still be treated as the world's sole superpower despite the dramatic attack, and it would still be wise to toe previous lines.
As things happened, the United States attacked Afghanistan. This proved to the international community that the US could still act on an international scale, and it made it look to the people that the government was acting to protect them. In point of fact, the invasion of Afghanistan didn't really make us incredibly safer. To be sure, it had some effect. But at the end of the day, it was a dramatic response to a dramatic provocation, designed to show everyone that the US was still on top.