Credibility and power do not merely stem from having major firepower, an extensive intelligence operation and a good economy. Credibility comes from trust.
Growing up my father once told me that as long as I was honest with him, he would would always be able to back me up, no questions asked. Being a parent, I think he would likely support me regardless, but his point was that a person's word is their bond. If they go back on that, who would rightfully trust them?
Now, no one believes that government shouldn't have secrets and that it should protect them. Yet when a President willfully misleads the public, especially on a matter as important as war, he undercuts his ability to function as a leader. Deception is often necessary in statecraft. Yet deception in the service of a war of choice is beyond the pale. Not only does it violate the trust of the people you swore to protect, but you hurt the country and its future relations as well. The President will leave office at some point, the country still must live with his legacy.
Recent reports have pretty much nailed down what many Bush-watchers believed to be the case, the White House willfully misled the American public in order to bolster support for his war in Iraq. A war they had every intention of waging, regardless of the circumstances.