Complicated Reasons

One thing that I have come to a conclusion about in Iraq is that there is not single reason we went to war.
There are lots of conspiracy theories involving Israel, oil, and the like, but hte fact of the matter is, several different groups pushed for an invasion for their own reasons.
(1) Some people were truly concerned about Saddam's perceived WMDs (Bill Buckley).
(2) Some wanted to end Saddam's reign because they believed it would be good for the Iraqi people and because they thought it would make up for abandoning the Kurds and Shiites in the early 90s (Paul Wolfowitz).
(3) Some wanted long-term American control over the Middle East for strategic reasons, including the ability to control the oil flow (James Woolsey). Note that the goal was strategic control of oil; not making the oil more available and cheaper in the short run, nor helping the oil companies, but gaining control of it as a strategic resource.
(4) Some believed it would improve Israel's strategic position (Richard Perle).
(5) Some wanted to take power in the new Iraq (Ahmad Chalabi).
(6) Although this may not have entered into the decision-making process fo anyone who officially influenced the administration, it's not unlikely that some wanted us to invade in order to strengthen the strategic position of the Iranians, which is why Iran, as I recall, gave us some logixtical help or something when we originally invaded.
(7) Some wanted us to take steps that they believed would help to lead us to Armageddon (Hal Lindsey).
(8) Some may have wanted us to keep Iraq, and by extension, other oil-producing nations, from going Euro and weakening the dollar (no particular name comes to mind).

In any case, I doubt that there was a unified conspiracy as much as there have been lots of people both before and after the invasion, attempting to carry out their agendas in the war.