Moments after planes crashed into the World Trade Center and the Pentagon, lawyers in the Justice Department's elite Office of Legal Counsel began crowding into the office of one of the agency's newest deputies, John C. Yoo, to watch the horror unfold on his television set. "We all stood around watching this event and he just seemed very calm, like he wasn't going to let these terrorists stop him from doing his work," recalled Robert J. Delahunty, a friend of Mr. Yoo's who worked in the office. Fearful of another attack and told that all "nonessential personnel" should evacuate, Mr....