Congress isnt responsible. They were given incorrect information and they made their decision off that information.
As was Bush. But ultimately, Congress IS responsible because they have the power to go to war, not Bush.
You also have to remember that the Head of the CIA practically hates Bush. It would not make very much sense for him to be collaborating with him, would it? Rather, either the CIA did it on purpose for their own agenda, or the CIA themselves were misinformed.