Unlike Iraq, Al Qaida had vividly demonstrated that it was an imminent danger (what phrase would be appropriate for an imminent danger that comes to pass? Whatever, it was proven to be a danger.). Or rather, in this scenario, somebody was demonstrated to be a real danger, and that's a key difference.
You have hit on the core of the entire Bush policy. After September 11, it is no longer acceptable to wait until after the attack to do something about it. This is a radical change in conventional thinking.
There is a word for an imminent danger that has come to pass. It's called an attack. In the case of terror attacks, there ARE no reliable signs of "imminent danger." There are no troops lining up on borders, no missiles being rolled out, no aircraft warming up on runways. There are no diplomatic warnings, no declarations of war, nothing. Look at all the evidence that was collected after the fact on the Sept. 11 terrorists. Much scorn was heaped on the FBI and CIA for missing the threat ahead of time, for not connecting the dots. Look at the collected evidence again. Would this have justified arresting these people and holding them? For what? Taking flying lessons? The ACLU would be all over the FBI like a cheap suit. Now, look at the collected evidence again. Maybe it would be enough to arrest the terrorists. But to convict them?
Now.
Would the evidence of a threat by the 9/11 terrorists, accumulated prior to their actual attacks on New York and Washington, have been sufficient to justify an attack on Afghanistan?
The level of proof demanded by the antiwar people of an "imminent threat" can apparently be met by only one thing: an actual attack.
Think about this. No one has been able to describe any other realistic "evidence" that would serve. Can you?
Maybe some people want to keep it at this level. That justification of a military attack on another nation can only be warranted by real physical attacks on our people. Morally, at least, this keeps things crystal clear. But is this the way we want it to be?
To the President, and many others, this is not acceptable any more, not when nineteen guys and bunch of boxcutters can wreck part of Manhattan and Washington DC, and kill 3,000 Americans. If they had had access to a nuclear warhead or nerve gas, there is not the slightest doubt that they would have used them.
Bush realized this, and stated it in his State of the Union speech:
Some have said we must not act until the threat is imminent. Since when have terrorists and tyrants announced their intentions, politely putting us on notice before they strike? If this threat is permitted to fully and suddenly emerge, all actions, all words, and all recriminations would come too late. Trusting in the sanity and restraint of Saddam Hussein is not a strategy, and it is not an option.
Acting on the best evidence available, Bush and his allies were unwilling to risk the consequences of what could happen if the information was accurate.