For a year, the Trump Justice Department has been on an odd mission: to assist a mysterious former FBI informant with ties to Russian intelligence who ended up in prison for passing disinformation about Joe Biden to the bureau. His crime deeply affected American politics. The false claim he slipped to the FBI—that Biden and his son Hunter each were paid a $5 million bribe by a Ukrainian energy company—became the main evidence in the House Republicans’ reckless and ill-fated impeachment drive against the 46th president.