British Prime Minister Theresa May has announced it was “highly likely” Russia was behind a nerve agent attack that left a Russian double agent and his daughter fighting for their lives.

Sergei Skripal, a retired Russian military intelligence officer living in Britain, collapsed March 4 on a Salisbury park bench, along with his daughter. For about a decade, Skripal spied for Britain’s foreign intelligence service, MI6, while working inside Russian intelligence. The Russian government arrested him for espionage in 2006, then transferred him to Britain in 2010 in a Cold War-style “spy swap.”

This isn’t a spy novel, but it sounds like one

The military-grade chemical attack on Skripal, which left hundreds of people in Salisbury exposed, has a depressing familiarity — in 2006, a dissident former Russian intelligence officer, Alexander Litvinenko, died after drinking tea laced with radioactive polonium in a London hotel. A British government inquiry into Litvinenko’s death concluded that it was a “probably” a hit job carried out by Russia’s FSB security service, with the approval of Russian President Vladimir Putin.

Russian state assassinations are not a recent development. The Kremlin has a long tradition, stretching back a century, of eliminating political “dissidents” at home and abroad — often in agonizing ways — to send messages to other political opponents.

Cold War histories tend to discuss KGB covert action — including assassinations — in passing, if at all. My research on intelligence and Cold War superpowers, drawing on previously classified KGB material, reveals the importance that successive Soviet leaders attached to “liquidating” traitors.

The sword and the shield 

Intelligence services played a central role in Soviet Russia from the outset. After the Bolsheviks seized power in Russia in 1917, the Soviet secret police, the Cheka — later known as the NKVD and KGB — became the Communist Party’s “sword and shield”: shield to defend the Russian Revolution; sword to smite its foes.

In the late 1930s, Joseph Stalin’s NKVD carried out the greatest peacetime repression in modern European history, the “Great Terror.” NKVD victims during the Great Terror likely number in the hundreds of thousands, shot in the back of the head, or sent to certain deaths in gulag prison camps.

Here are three examples of how Soviet, and now Russian, intelligence services sought to eradicate “enemies of the state”:

1) Eliminating a nemesis

Under Stalin’s rule, the NKVD went to elaborate lengths to track down and kill his ideological nemesis, the communist “heretic,” Leon Trotsky. At the outbreak of World War II, Stalin saw eliminating Trotsky as a higher priority for the NKVD than collecting intelligence on Adolf Hitler’s Germany.

Eventually an NKVD assassination unit, the “Administration for Special Tasks,” located Trotsky in exile in Mexico. An initial team, led by a Lithuanian assassin, Iosif Grigulevich, who later fabricated a career as a Costa Rican diplomat, failed to do the job.

Another NKVD deep-cover agent, Ramón Mercader, spent several months seducing and befriending his way into Trotsky’s villa near Mexico City. In August 1940, Mercader found himself alone with Trotsky and killed him with an ice pick to the head. As well as silencing Trotsky, the killing was intended to send a clear message to other “traitors.”

Mercader was caught, arrested and imprisoned in Mexico. After his release in 1960, he was awarded the title “Hero of the Soviet Union.” To this day in Russia, “sending a Mercader” means despatching an assassin.

2) Silencing dissidents

The NKVD also experimented with elaborate methods of killing “enemies of the people,” often in painful and hard-to-trace ways. In the 1930s, it established a sophisticated medical section, the Kamera, which developed lethal toxins.

One of its inventions was a gel that induced a heart attack when rubbed on a victim’s skin. Years later, in the early 1970s, the NKVD’s successor, the KGB, managed to rub such a gel on the skin of a dissident Soviet writer, Alexander Solzhenitsyn, but it failed to kill him, only making him severely ill.

3) Poisoned gun and umbrella

In 1961, a KGB assassin, Bogdan Stashinsky, defected to the West, revealing his Kremlin-sanctioned missions to kill traitors using a poisoned gun. In 1978, the KGB went to similarly elaborate lengths to assassinate a Bulgarian emigre playwright, Georgi Markov, an outspoken Soviet critic living in London. According to a high-level KGB officer at the time, KGB Chairman Yuri Andropov, who later became the Soviet leader, personally authorized the KGB to assist the Bulgarian security service with the hit on Markov.

The KGB’s technical department devised a sophisticated murder weapon — an umbrella capable of firing a poisoned pellet, which they tested on a prisoner sentenced to death. As Markov walked across London’s Waterloo Bridge, an assailant stabbed him with the lethal umbrella, implanting a pellet laced with ricin. Markov died an agonizing death four days later.

“Active measures” are part of Putin’s arsenal

News headlines today are dominated by Russian “active measures” aimed at interfering in the 2016 U.S. election. In fact, in the KGB tradition, Russian active measures cover a wide spectrum of “political warfare” activities, which seek to influence world events.

These range from “influence operations,” like election interference, to various degrees of physical violence, including, at the most extreme, assassination. Litvinenko’s polonium poisoning in 2006, and now apparently the chemical attack on Skripal, are throwbacks to earlier KGB active measures — “wet jobs” on dissidents like Markov.

It is unsurprising that the Kremlin should undertake killings in the KGB’s manner, given Putin’s former KGB career. Russia’s current intelligence services, the FSB and SVR, have expressly embraced the KGB’s Cold War methods. In 2006, Russia passed a law permitting extrajudicial killing of “extremists” abroad.

In recent years, a number of Putin critics have died under suspicious circumstances — in poisonings, shootings and mysterious ways. The British prime minister has stated Russia is highly likely behind the attack on Skripal, but she left open whether it was Kremlin-directed or whether Russia has lost control of its nerve agent.

Unlike the Litvinenko case, we do not know whether the long arm of the Kremlin lies behind the chemical attack. However, nerve agents are highly controlled substances, and orchestrating their use in a foreign country is a sophisticated operation, strongly suggesting state involvement.

A likely motivation, in this case, is revenge for Skripal’s espionage for MI6. Given the long Soviet history of eliminating “traitors,” now inherited by Russia, it would not be surprising if the nerve agent is the latest example of this horrific tradition. No doubt we shall soon discover more about the modern-day Mercader.

Calder Walton (@calder_walton) is an Ernest May Fellow in History and Policy at Harvard’s Kennedy School, where he is writing a book on intelligence, superpowers and the Cold War.