The Taliban were sheltering Al-Quaeda, which were admittedly the masterminds of a direct attack on us. We had every right to go in after them when the existing government protected them, therefore, in defacto alliance with them. The civil war and Northern Alliance worked in our favor, but we had to go in even if they hadn't been there. If we had captured Bin Ladin and broken his organization, we could have left immediately and left the Northern Alliance in charge, a useful ally. There would have been no reason to stay.

Iraq was different. It was strategic, rather than tactical. We had no quarrel with Saddam, he was simply in the right place at the right time, so we had to pick a fight. He offered a springboard to threaten the entire middle east, plus he had oil. We moved in because he was militarily and economically weak, and because his neighbors and his people hated him. We needed to be there not to solve a problem, but to prevent any future problems in the area.

Yeah I know, WMDs blah blah. I'll be charitable and concede maybe we did believed he had WMDs. I doubt that now, but I think we would have gone in whether or not we believed it. Cheney himself has flat out admitted that. Iraq was the keystone state, an American presence there would simultaneously threaten all the hostile states, provide support for all our allies in the region, and keep Iran hemmed in. And Iraq had access to the sea! It was the perfect chess move.

We had to go into Iraq for strategic AND economic reasons. No more oil blackmail form the Arabs! Unfortunately, the American people would never have stood for that. The USA does not attack foreign countries just so we can intimidate their neighbors and steal their oil. That's the sort of thing the Nazis and Communists did, not us. It's called flat out agression, and America hadn't done it since we invaded Mexico in the 1840s.

As far as nation-building was concerned. The Republicans sneered at Clinton for trying it in the Balkans, and now Bush found himself forced to do it in Iraq. They never really tried it in Afghanistan, though, which is why we let our initial advantage there slip through our fingers.