Then-Deputy Attorney General Rod Rosenstein was determined to keep his big secret about a pending special counsel right up to the day his top candidate, Robert Mueller, sat down with President Trump in the Oval Office. “The boss and his staff do not know about our discussions,” Mr. Rosenstein said in an email to Mr. Mueller on May 12, 2017. Mr. Rosenstein’s “boss” was an apparent reference to Attorney General Jeff Sessions, who had stepped away from any role in the Russia probe. This meant that Mr. Trump didn’t know either. The email was one of a thread of messages...