Why did we go to war with Iraq? According to Joshua Marshall of Talking Points Memo:

"This war isn't really about Iraq or deposing Saddam or even eliminating his WMD, though each of those are important benefits along the way. Nor is it something so mundane as a 'war for oil.' The leading architects of this war in and out of the administration see this war, and have pursued it, as an opening blow in a far broader war against political Islam. They see it as the first in a series of wars and near-wars which will lead eventually to the overthrow of most of the current governments in the Middle East, the establishment of western-oriented democracies throughout the Arab world, and the destruction of nothing less than the political world of Islamic fundamentalism."

You can read a short discussion of these ideas on this Talking Points Memo post, or a longer discussion in this article from the Washington Monthly.