Just before Christmas 2015, the British intelligence operative Christopher Steele emailed a report to private clients that included an American lawyer for a Ukrainian oligarch. The title of the dossier was “FIRTASH Abortive Return to Ukraine,” and it purported to provide intelligence on why the energy oligarch Dmitri Firtash tried, but failed, to return to his home country of Ukraine. “FIRTASH’s talk of returning to Ukraine a genuine ambition rather than merely a ruse to reveal Ukrainian government’s hand. However the oligarch developed cold feet upon the news of a negative reception at Boryspil airport,” Steele reported on Dec. 23,...