Never ignore the warnings of refugees and defectors from revanchist authoritarian regimes. That’s the lesson the British security establishment is learning the hard way in the wake of last week’s poisoning of a Russian ex-spy, Sergei Skripal, on U.K. soil. The nerve-agent attack in Salisbury, England, left Skripal and his daughter Yulia in critical condition. Hundreds of innocent Britons may have been exposed to the toxin, and it caused at least one to fall ill.

The Kremlin’s fingerprints are all over this act of terror. Prime Minister Theresa May has issued an ultimatum to Moscow demanding it to account for the episode—or else. But first, the British prime minister might ask herself how it was that Russian operators came to conclude that they could get away with so brazen a violation of U.K. sovereignty. The simple answer is: because it worked the last time.

Few can fail to notice the similarities between the Skripal case and 2006’s polonium poisoning of Alexander Litvinenko, another former Russian spy who had taken refuge in Britain. What most may not remember, however, are the lengths to which Britain’s political class went to avoid airing out the facts of that case. Although investigators charged one of the suspects in the Litvinenko murder (in absentia) in 2007, the government long resisted the launch of an independent inquiry.

That was back when Britain and other European powers, following Barack Obama’s footsteps, were keen to appease Moscow. It was necessary to show “flexibility,” to let old bygones be bygones. Never mind that Litvinenko’s killers had deployed a substance that emits 166 quadrillion radioactive alpha particles per second. In 2013, a senior official in David Cameron’s government conceded in a letter to Britain’s High Court of Justice that “international relations”—that is, relations with Moscow—“have been a factor in the government’s decision-making.”

That senior official was then-Home Secretary Theresa May.

For years, Litvinenko’s widow, Marina, pressed the government to launch a commission to investigate her late husband’s death, but to no avail. It wasn’t until 2014 that Mrs. May reversed course, and then only after Russian-backed rebels downed Malaysia Airlines Flight 17 over Ukraine. The Litvinenko Inquiry got underway the following year.

As Marina Litvinenko told me in an interview in 2014, British leaders “put good relations with Russia in top priority. But the Russians have been trained in a different way. They’re not from Oxford, or from Cambridge. They’ve got a different agenda. They’ve been trained by the KGB .  . . Every time you cede more, they will try to catch you in a weaker position. If you say, ‘Excuse me, I did something wrong,’ they don’t appreciate it—they say, ‘OK, now I will make it worse for you.’ ”

She told you so.