however to be impeached, one needs to have performed an offense to the constitution or law. As I understand (and I am far from an expert) the constitution gives the president (as commander in chief) the ability to send troops to battle regardless of reason. Furthermore, I believe that the president does not need congress to approve it. Congree basically only approve the budget, and give their nod that they approve of the plans.
The fault is that he sold the war to everyone as a search for WMD's, which was definitely part of the story but not much of it. As I see it, he took part of the truth and used that to sell his agenda, and kept many of the real reasons locked away... I find it hard to believe any politician, or anyone coming to my door to try to convince me to back any political purpose.