I don't see why we couldn't have stopped at killing or capturing those responsible and stopping there. What it turned into was war against an indefinite, amorphous opponent. I think hardly anyone had or has an issue with our excursion into Afghanistan itself, but rather, the way it was carried out.
I might be falling right in line with the Paulbots here, but I think the vast military operations (placed on the credit card) under Bush and continued to a lesser extent under Obama are a result mostly of the influence of the military industrial complex.