"He was thinking about invading Iraq in 1999," said author and Houston Chronicle journalist Mickey Herskowitz. "It was on his mind. He said, 'One of the keys to being seen as a great leader is to be seen as a commander-in-chief.' And he said, 'My father had all this political capital built up when he drove the Iraqis out of Kuwait and he wasted it.' He went on,
'If I have a chance to invade…, if I had that much capital, I'm not going to waste it. I'm going to get everything passed that I want to get passed and I'm going to have a successful presidency.'"Bush wanted a war so that he could build the political capital necessary to achieve his domestic agenda and become, in his mind, "a great president." Blair and the members of his cabinet, unaware of the Herskowitz conversations, placed Bush's decision to mount an invasion in or about July of 2002. But for Bush, the question that summer was not whether, it was only how and when. The most important question, why, was left for later.
Eventually, there would be a succession of answers to that question: weapons of mass destruction, links to Al Qaeda, the promotion of democracy, the domino theory of the Middle East. But none of them have been as convincing as the reason George W. Bush gave way back in the summer of 1999.
http://www.tompaine.com/articles/20050620/why_george_went_to_war.php