I think Salon's Gary Kamiya is on to something in this analysis of why Bush hasn't been impeached. But I'm not convinced 9/11 even enters into it:

It is arguable that if there had been no 9/11, Bush's fraudulent case for war really would have resulted in his impeachment -- though this is far from certain. But 9/11 did happen, and as a result, large numbers of Americans did not just give Bush carte blanche but actively wanted him to attack someone....

To this day, the primitive feeling that in response to 9/11 we had to hit hard at "the enemy," whoever that might be, is a sacred cow. America's deference to the shoot-first-and-ask-questions-later approach is profound: It's the gut belief that still drives Bush supporters and leads them to regard war critics as contemptible appeasers. This is why Bush endlessly repeats his mantra "We're staying on the attack."

The unpleasant truth is that Bush did what a lot of Americans wanted him to. And when it became clear after the fact that Bush had lied about the threat posed by Saddam Hussein, it made no sense for those Americans to turn on him. Truth was never their major concern anyway -- revenge was. And if we took revenge on the wrong person, well, better a misplaced revenge than none at all.

But if Bush had led us into a disastrous war in Iraq without the pretext of 9/11 -- which really might have happened, given the monomania of the neocons and Cheney, as well as Rove's lust for wedge issues -- would Bush really be at risk of impeachment? Remember, we didn't impeach Reagan for Iran-contra, either, and that was before 9/11. And in the post-Vietnam War era we've probably never reached the heights of fervor to crush an enemy that we reached around the time of the first Gulf War -- and the proximate cause for our outrage was a mere invasion of Kuwait.

If we'd invaded Iraq without a 9/11, it would have been because Bush had successfully trumped up the Saddam threat and persuaded the American people that war was vitally necessary. Americans are not very hard to persuade in matters like this when the president is a tough-talkin' Republican -- think Grenada and Panama. If Bush had successfully sold us the war, wouldn't a lot of Americans, in their guts, still be thinking we'd had no choice but to take action against an imminent threat? Wouldn't we be pretty much where we are now?

Kamiya's on more solid ground here:

There is an ancient human deference to The Strong Man Who Will Defend Us, an atavistic surrender to authority.... Even when it is unequivocally shown that a leader lied about war, as is the case with Bush, he or she is still protected by this aura. Going to war is the best thing a rogue president can do. It's like taking refuge in a church: No one can come and get you there. There's a reason Bush kept repeating, "I'm a war president. I'm a war president." It worked, literally, like a charm.

Without a 9/11, I think Bush probably would have done that anyway: declared that he'd Defend Us against the Saddam regime and painted himself as The Strong Man.

Of course, you have to be the right kind of war president -- a Republican. (Or possibly a Joe Lieberman/Zell Miller Democrat.) It's easy to imagine calls for Bill Clinton's impeachment if, somehow, Haiti or Somalia had led to 3,000 troop deaths and a quagmire with no end in sight. As for Al Gore, if 9/11 had happened on his watch, I think the public would have rallied around him -- but Republicans would have started working on undermining faith in his leadership within days of the attacks, if not hours. Even a quick victory in Afghanistan and a few other successes against jihadists wouldn't have been enough to silence Republicans, who'd be pointing out that all Islamicist terrorism wasn't driven from the face of the earth instantaneously. Gore wouldn't have been impeached; someone (John McCain? Jeb? W in a rematch?) would have probably beaten him in '04 by painting him as weak on terror. The "ancient human deference to The Strong Man Who Will Defend Us" would have given way to the modern American stereotype of all Democrats, even one who may have driven the Taliban from power, as sandal-wearing hippies.