SOFIA, Bulgaria — The Russian assassin used an alias, Sergei Fedotov, and slipped into Bulgaria unnoticed, checking into a hotel in Sofia near the office of a local arms manufacturer who had been selling ammunition to Ukraine. He led a team of three men. Within days, one man sneaked into a locked parking garage, smeared poison on the handle of the arms manufacturer’s car, then left, undetected, except for blurry images captured by surveillance video. Shortly after, the arms manufacturer, Emilian Gebrev, was meeting with business partners at a rooftop restaurant when he began to hallucinate and vomit. The poisoning...