He's too gullible:
I felt confident that Bush would work with the international community. I took the President at his word. We were told that any course would lead through the United Nations, and that war would be an absolute last resort. Many people I am close with, both Democrats and Republicans, who are also close to Bush told me unequivocally that no decisions had been made about the course of action. Bush hadn't yet been hijacked by Wolfowitz, Perle, Cheney and that whole crew. Did I think Bush was going to charge unilaterally into war? No. Did I think he would make such an incredible mess of the situation? No. Am I angry about it? You're God damned right I am. I chose to believe the President of the United States. That was a terrible mistake.
—John Kerry (source)
He believed the president. Idiot.