"I wonder if this war would have happened had 9/11 never happened, had the only difficulty with Iraq been the possibility of Iraq having or developing weapons of mass destruction."
According to Rumsfeld, Cheney, Rice, Wolfowitz, et al on their web site (Project for a New American Century), yes, Iraq would have been invaded. Here it their web page - you can find their position advocating that back in 1998:
The groups policy paper demanding American global dominance, explicitly opposing the allowing of any other country (even allies) from attaining a position able to challenge the U.S military superiority, was read by Bush, slightly modified, as the State of the Union address post-9/11.
Without 9/11, it might have waited until the second term, allowing a greater build-up of fear - or at least until the fourth year of Bush's first term. But the statement of intent was (and remains) clear.
It's a great resource to find out what the administration's future plans are. It surprises me that anyone is ever surprised when the U.S administration carries them out, as they're very publicly (and proudly) visible.