I put this answer on an article on Tom Delay.com. I liked the way it sounded so I expanded and am repeating it here:
I think people are supportive of the troops in Iraq, but are upset that we did not have sufficient deliberation before the invasion. Our justification for the invasion was that Hussein was not decreasing his stock piles of WMDs fast enough. That is a weak justification for the violence that ensued. There is also the question of whether or not Iraq was the best target. Personally, I think stopping the genocide in Sudan would have been a better move. There are arguments that Iran was a greater threat to world stability than Iraq. The energy wasted in Iraq would have been better spent trying to counter Iran.
Americans hate the fact that we aren't on the moral high ground with the invasion.
Because we did not go through a sufficient process of deliberation, we ended up leaving our political flanks open, and the left was able to attack with a vengeance.