The Federal Bureau of Investigation has had its worst week in modern history. The Justice Department’s inspector general found that the bureau had deceived a federal court and abused Americans’ civil liberties. It was equally humiliating for the crew that gulled the FBI into its excesses: Fusion GPS, Christopher Steele and their media acolytes.

Fusion is the opposition-research firm the Democratic National Committee and the Hillary Clinton campaign hired in 2016 to kneecap Donald Trump. Fusion in turn hired Mr. Steele, a British...