For more than four decades before Donald Trump assumed the presidency, the FBI director was a position above politics. A new president might choose a political ally as attorney general, but the FBI director was different. An FBI director appointed by Richard Nixon also served under Presidents Gerald Ford and Jimmy Carter. Carter’s choice remained on the job deep into Reagan’s second term, when Reagan moved him to head the CIA. Reagan’s FBI appointee served through the George H. W. Bush presidency and into the Bill Clinton administration. Clinton fired the inherited official—the first time a president ever fired an...