Bush's advisors are neo-cons. His doctrine is the neo-con doctrine. Irag as a base from which we can project power was always a keystone of that doctrine.
Saudi Arabia was/is not on the list of state sponsors of terrorism. We don't invade states just because their nationals are involved in terrorist acts. The shoe bomber was a citizen of the UK. Saudi Arabia has been a victim of AQ terrorist acts, not the state sponsor of them.
Iran is a state sponsor of terrorism, but it has not invaded two of its neighbors. Iraq was a more immediate threat. Moreover, Iran is not as easy a target. It is a vast country of 1.5 million km (four times bigger than Iraq) with over 66 million people (almost three times Iraq). There is no doubt that we will have to make some hard decisions on Iran and its nascent nuclear program. Being next door with a sizeable military capability gives us more options.
Bush's advisors are neo-cons. His doctrine is the neo-con doctrine. Irag as a base from which we can project power was always a keystone of that doctrine.
Rather simplistic explanation of a complicated problem. 9/11 was the catalyst that forced us to deal with global terrorism, which cannot exist without state sponsorship. It there had been no 9/11, there would not have been an invasion of Iraq, neocon recommendations to the contrary.