I'm not really an expert so I don't keep links handy. A duckduckgo search of "bush iraq invasion before 9/11" [0] yields:
http://www.cnn.com/2004/ALLPOLITICS/01/10/oneill.bush/
> (CNN) -- The Bush administration began planning to use U.S. troops to invade Iraq within days after the former Texas governor entered the White House three years ago, former Treasury Secretary Paul O'Neill told CBS News' 60 Minutes.
> "From the very beginning, there was a conviction that Saddam Hussein was a bad person and that he needed to go," O'Neill told CBS, according to excerpts released Saturday by the network. "For me, the notion of pre-emption, that the U.S. has the unilateral right to do whatever we decide to do, is a really huge leap."
http://www.cbsnews.com/news/bush-sought-way-to-invade-iraq/
> He got briefing materials under this cover sheet. "There are memos. One of them marked, secret, says, 'Plan for post-Saddam Iraq,'" adds Suskind, who says that they discussed an occupation of Iraq in January and February of 2001
> Based on his interviews with O'Neill and several other officials at the meetings, Suskind writes that the planning envisioned peacekeeping troops, war crimes tribunals, and even divvying up Iraq's oil wealth.
For reference, the towers fell on Sept 11, 2001.
For the rationale my understanding mostly derives from various reddit threads (e.g. [1]), a friend who majored in criminology, and this fantastic overview of Iraq at Wait but Why [2].
[0] https://duckduckgo.com/?q=bush+iraq+invasion+before+9%2F11
[1] https://www.reddit.com/r/changemyview/comments/1lx1s7/oil_pl...
[2] http://waitbutwhy.com/2014/09/muhammad-isis-iraqs-full-story...