Donald Trump’s sinister affinity for Russia’s president, Vladimir Putin, has long been the subject of intense speculation. Former KGB officers claim Trump was recruited in Moscow in 1987 and cultivated as an asset in the years prior to his 2016 US election victory. Two retired Russian spies weighed in again last month, alleging that the then 40-year-old Trump, codename “Krasnov”, was personally compromised in an “active measures” operation and has secretly danced to Putin’s tune ever since. Nothing is proved and all is denied. Yet the so-called Steele dossier, compiled by a British MI6 ex-spy chief, the FBI’s Mueller report,...