Prime Minister Theresa May announced Wednesday that her government will expel 23 Russian diplomats from Britain. It will be the biggest expulsion of Russian diplomats from the country since 1985 — marking a return to the large-scale diplomatic ejections that took place during the Cold War.

May's announcement came after the poisoning of former Russian double agent Sergei Skripal, 66, and his daughter, Yulia, 33, in the English town of Salisbury. British authorities have concluded that the two appeared to have been targeted for assassination last week with the nerve agent Novichok, which traces back to the Russian state.

A midnight deadline for an explanation from Moscow passed with little Russian reaction. Speaking in Parliament after her regularly scheduled question time, May said Moscow had instead “treated the use of a military-grade nerve agent in Europe with sarcasm, contempt and defiance.”

Though Britain has expelled Russian diplomats in recent years, the scale of this expulsion is exceptional. May said it would be the largest such action in 30 years.

Additionally, as the Russian diplomatic presence in Britain is smaller than the Soviet Union's was during the Cold War, it will have a bigger effect, noted Calder Walton, a British lawyer and author of “Empire of Secrets: British Intelligence, the Cold War, and the Twilight of Empire.” May's government lists 58 Russian diplomats in Britain, which means that almost 40 percent of them are being expelled.

 The last ejection on a similar scale occurred in 1985, when Britain expelled 25 alleged Soviet spies after Oleg Gordievksy, a senior KGB agent, defected to Britain. That incident sparked tit-for-tat diplomatic moves, ultimately resulting in the removal of 31 Soviet agents from Britain and the same number of Britons from the Soviet Union. At the time, there were concerns that the expulsions could derail bilateral relations just as the Cold War was starting to thaw.

“Never engage in a pissing match with a skunk: He possesses important natural advantages,” was the advice that Britain's then-ambassador in Moscow, Bryan Cartledge, offered in a cable sent home that was recently declassified.

The largest and most infamous expulsion of spies in British history occurred in 1971, when Britain expelled 90 Soviet diplomats accused of espionage and barred 15 Soviet officials from traveling to the country. The move, which also came after the defection of a top KGB officer, took place when the Soviet Union was reported to have 550 diplomats in Britain — meaning that nearly 1 in 5 Soviet diplomats were forced to leave the country.

Moscow responded to those removals, still thought to be the largest single expulsion of intelligence officials in history, with intransigence. Soviet Foreign Minister Andrei Gromyko met privately with Foreign Secretary Alec Douglas-Home at the United Nations in New York and admonished him for the “hooligan-like acts of the British police,” saying London was trying to distract from “the bottle necks, rents and tatters of their own policies,” according to documents published decades later by Britain's Foreign Office.

Indeed, Britain was a key U.S. ally during the Cold War, making it both a target of espionage and an active participant in it. However, the collapse of the Soviet Union didn't necessarily improve diplomatic relations between Britain and Russia — in 1996, London expelled four Russian diplomats after Moscow claimed to have uncovered a British spying ring and expelled the same number of British diplomats.

Later, London's burgeoning status as a destination for wealthy Russian emigres, many of whom were vocal critics of Russian leader Vladimir Putin, led to further diplomatic tension. In 2007, after Moscow refused to extradite the suspects in the assassination of Putin critic Alexander Litvinenko on British soil — again, a poisoning quickly traced back to Russia — Britain responded by expelling four Russian diplomats.

“This is a situation the government has not sought and does not welcome,” Foreign Secretary David Miliband said at the time. “But we have no choice but to address it.”

Three years later, Britain expelled another alleged Russian spy; intelligence sources told the British press that the spy had overstepped the unwritten rules of espionage.

May's expulsion of Russian diplomats from Britain will probably be felt keenly in what is one of Moscow's most high-profile diplomatic offices. It could have a bigger effect on staffing than President Barack Obama's 2016 decision to expel 35 "intelligence operatives" in response to alleged Russian interference in the U.S. presidential election — a move that cut Russia's total staffing in its U.S. diplomatic missions to 455.

But the big question is whether the old tactic of diplomatic expulsion is the right one for a newly antagonistic Russia, especially given the likelihood of an in-kind retaliation by Moscow. Walton said it was unclear how a Cold War tactic such as expulsions would work in an age “when espionage is conducted in cyberspace.”

He added, however, that there was still a human element to most online spying. “We are discovering more and more that even in the age of cyberespionage, there is still an important role for traditional, age-old, human espionage: recruiting agents,” Walton said.

Many had hoped that London would combine a traditional response to Russian espionage with more unorthodox methods, including financial measures against the Kremlin-linked oligarchs who now call London home and even some kind of cyberattack. Though May hinted at both, her statement to Parliament on Wednesday hewed to the tried-and-tested: expulsions.

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