UK Prime Minister Theresa May said her country will expel 23 Russian diplomats on Wednesday — which would be the country’s largest removal of foreign officials in more than 30 years.

Here’s why: On March 4, Sergei Skripal, a former Soviet and Russian spy, was found unconscious on a bench alongside his daughter Yulia. According to May, they were poisoned with a nerve agent known as “Novichok” in Salisbury, Southern England, where he lives. Both Skripal and his daughter are still in the hospital and are reportedly in dire condition.

Novichok is part of a group of nerve agents considered to be the most lethal in the world, and the Russian government has been producing it for decades.

Two days ago, May said it was “highly likely” Russia was behind the mysterious chemical attack and requested that the country help her government understand exactly what happened.

But it appears Russia didn’t comply with her request, and now she says “there is no alternative conclusion other than that the Russian State was culpable for the attempted murder of Mr. Skripal and his daughter — and for threatening the lives of other British citizens.”

May also suspended all high-level contact between the UK and Russia with no indication of when they will resume again.

Even before May’s announcement, experts I spoke to thought Moscow was at fault. “This is a classic Kremlin playbook [move],” Rachel Rizzo, a European security expert at the Center for a New American Security think tank, told me on Monday.

Moscow denies any involvement in the attack and said any effort to remove Russian diplomats from London would be met with a serious response. “The British side should be aware of that,” the Russian Embassy in the UK tweeted. In a separate tweet, the embassy relayed a statement from Kremlin spokesperson Dmitri Peskov where he took a shot at British media for fingering Moscow for the attack.

Kremlin spox Peskov: Russia, having nothing to do with Sergei Skripal poisoning, is ready to cooperate with UK, but sees no reciprocity. UK media proved their “quality” – no more trust in them. pic.twitter.com/YM0fGt12L5 — Russian Embassy, UK (@RussianEmbassy) March 14, 2018

But Russia has a history of targeting people in the UK that the Russian government believes are enemies of the state — and it looks like that just happened again.

Who is Sergei Skripal?

Skripal may be one of the most important turncoats you’ve never heard of.

He was a member of a Russian intelligence service, known as the GRU, who was convicted in 2006 of handing over the names of 300 Russian agents to the British. Skripal admitted to helping UK spies for at least a decade.

But in 2010, the US and Russia swapped the largest number of spies since the end of the Cold War, and Skripal was among them. He then settled into a fairly modest life in a Salisbury cul-de-sac. Some unconfirmed reports suggest Skripal received official warnings that his life was in danger before the March 4 attack, but he apparently changed nothing about his lifestyle.

Other unconfirmed reports say Skripal had a large amount of money in his bank account, and that there are phone records connecting him to a British spy in Estonia, perhaps the agent that recruited him to work for the UK.

It’s unclear if Skripal ever spoke out against Putin or the Russian government, and it’s especially unclear why Russia would target him now. But Evelyn Farkas, a former top Pentagon official focused on Russia, thinks Skripal may have been on a Russian list for some time: “This looks to me like he’s on a list of former spies and former Kremlin cronies who Moscow likes to eliminate as a sign that you have to stay loyal.”

That list, sadly, is seemingly quite long.

“We’ve seen this happen before, and we’ll see it happen again”

The attack on Skripal isn’t an isolated incident. Russia has a longstanding campaign to identify and kill Russian dissidents living in the UK.

US intelligence links at least 14 deaths in the UK to Russia, including outspoken oligarchs and journalists. “We know the Russians have an active program of killing people in the UK that they don’t like,” a British intelligence officer told Steven Hall, a former CIA official focused on Russia, according to BuzzFeed News.

The Skripal case has already drawn comparisons to one of those murders: that of Alexander Litvinenko, a former Russian spy and defector to the UK. In 2006, two Russian agents put polonium-210 — a highly radioactive chemical — in Litvinenko’s tea at a London hotel bar. It took weeks for Litvinenko to die, and he blamed Putin for orchestrating the attack.

“You may succeed in silencing one man,” Litvinenko said from his hospital bed, “but the howl of protest from around the world will reverberate, Mr. Putin, in your ears for the rest of your life.” Russia continues to deny any involvement in Litvinenko’s death.

There are other cases as well. In 2013, Boris Berezovsky, then a Russian oligarch, died on a bathroom floor in his home outside London. Berezovsky was a Putin critic — at one point, he called for a coup against Putin — who blamed the Kremlin for Litvinenko’s death. Berezovsky eventually tried to go back to Russia, but he died before Putin granted his request to return.

That, in part, is why analysts suspected Russia from the start. “We’ve seen this happen before, and we’ll see it happen again,” Rizzo told me on Monday. “The question is: What does the West do about it?”