There never really was one reason, though some people think we invaded for oil or retribution or Texas bloodlust. The Oil I'm really least conviced about; here's part of what I think the real cause was.
- George W. Bush was utterly terrified after the 2001 terrorist attacks. He had a large staff, that was partially made up of tactical experts and partly made up of his best friends, and the consensus among them was that a retaliatory strike on someone was better than no strike at all.
- Some members of the US intelligence community felt that Iraq was trying to develop WMDs. What made them think that, and why they felt it was more critical than, for instance, North Korea's already existing weapons, I will never know.
- The group responsible for the 2001 attacks was believed to be in Iraq, as well as Iran, Afganistan, and Saudi Arabia. In fact, Osama Ben Laden was in fact a member of the Saudi royal family (a black sheep, of course). Since there was already some information about Iran, however, that was the primary target.
- I want to be clear here; The Bush administration most likely did not intend to misinform the public about goals of the Iraqi invasion; they may have said "We are launching forces to free Iraq, and some forces to fight the terrorists responsible for 9-11", but it was the news media, desperate for good news, that reported "We are launching forces to fight the terrorists in Iraq."
I am an American, and on the left end of the political spectrum, I'm sure. I feel that a peacetime military is a waste of money, and that the US should never act as the World Police. Perhaps the world does need police, but it's expensive and annoying. Let the Chinese do it, since they already own the US and make all our products. The US can sit back and gentrify like Sweden and Denmark.
As for the aftermath, I'm pretty sure that forcing someone to be in a democracy is impossible. They'll just vote it back to the way it was. Besides that, given that there are two main muslim sects, one of which usually outnumbers the other two to one, and both of which will vote each other into gas chambers...
What I'm saying is it's incredibly difficult to simply come into a country, blow it up entirely, then tell the surviving inhabitants (of which one in every ten lost someone they knew to misfired American missiles and such) that they are now in a democracy.