You can't impeach people like Rumsfeld, Wolfowitz, Rove, Feith, and Prof. John Yoo who have left office.
Instead, they should be investigated as quickly as possible next year and INDICTED for war crimes. The penalty under Title 18 of the U.S. Code is a fine, up to 20 years imprisonment or both.
Nothing like a "perp walk" for all the conspirators from Bush on down, and the prospect of some real "hard time" in a federal slammer to discourage such conduct in the future.