PRAGUE — Russia’s unraveling relations with the West took a dramatic turn for the worse on Thursday when the Czech Republic, furious over what it said were Moscow’s fingerprints on a military-style sabotage attack on a Czech weapons warehouse in 2014, ordered the expulsion of as many as 60 Russian diplomats.
The Czech move, announced a day after President Vladimir V. Putin of Russia warned that the West risked a “fast and tough” response if it interfered with his country, escalated not only a diplomatic crisis between Prague and Moscow but a wider showdown between Russia and NATO, of which the Czech Republic is a member.
With Russian troops massing near the border with Ukraine and President Biden taking a tough stand against the Kremlin, Mr. Putin on Wednesday bluntly warned the West not to test Russia’s resolve in defending its interests, telling it not to cross unspecified “red lines” that he said would be defined by Russia.
The slashing of staff at Moscow’s embassy in Prague does not directly challenge Russian security. But it will severely damage intelligence operations, something that Mr. Putin, a K.G.B. officer in Eastern Europe during the Cold War, views as vitally important.
It is the largest number of Russian diplomats ejected from a single country since President Ronald Reagan expelled 80 Cold War-era Soviet emissaries in 1986.
Michal Koran, a researcher in Prague at the Global Arena Research Institute, described the escalating row as “the most serious crisis in Czech-Russian relations” since the Soviet Union invaded what was then Czechoslovakia in 1968 to crush efforts by the leadership in Prague to create “communism with a human face.”
Russia withdrew its last troops from what is now the Czech Republic and other previously communist nations in East and Central Europe in the early 1990s. But it has long viewed the region as an area of special interest that owes Moscow gratitude, and a measure of deference, in return for the role played by the Red Army in liberating it from Nazi occupation.
The Czech president, Milos Zeman, is widely seen as pro-Russian but Prime Minister Andrej Babis and much of the opposition now tend to view Russia as an aggressive menace.
The Czech foreign ministry, in a statement on Thursday, said Russia must reduce the size of its embassy staff in Prague to correspond with the level of Czech representation in Moscow, which shrank to a skeleton staff after Russia on Sunday ordered out 20 Czech diplomats. Russia, which has used its Prague embassy as a center of espionage across the region, according to intelligence experts, previously had far more diplomats in the city than the Czech Republic had in Moscow.
The Czech ministry did not specify how many Russians would be expelled but the foreign minister, Jakub Kulhanek, said this week that bringing the two countries’ embassies to the same level would mean the departure of around 60 Russian diplomats. Prague, he said, had no choice but to slash Russia’s diplomatic representation unless Moscow reversed its decision to expel Czech diplomats, a demand the Russian government rejected as an impertinent ultimatum.
Russia’s Foreign Ministry said it would respond “soon” to the Czech announcement. It usually responds with what it calls “mirror measures,” normally a series of reciprocal expulsions. But with so few Czech diplomats left in Moscow — around five — this is now impossible, raising the possibility of a different form of retaliation.
The expulsion of the Czech diplomats from Moscow on Sunday followed the ejection on Saturday of 18 Russian diplomats from Prague. The rapid deterioration in relations was triggered by accusations that Russian military intelligence operatives were responsible for blowing up a Czech ammunition depot in 2014.
Czech authorities had been investigating the mysterious explosions, which killed two Czech workers, with little success for years. But they got what they say was a big break thanks to evidence uncovered in Britain after the 2018 attack with a nerve agent on a turncoat former Russian spy, Sergei V. Skripal, in the English town of Salisbury.
Two Russians identified by Britain as the main culprits in the Salisbury attack, both members of a military intelligence sabotage and assassination squad known as Unit 29155, turned out to be the same men Czech investigators had long suspected of involvement in the ammunition warehouse blasts but had not been able to identify.
Both men arrived in the Czech Republic under false names several days before the blasts and traveled to the site of the warehouse in Vrbetice, leaving on the day of the first explosion on Oct. 16, 2014.
Miroslav Mares, an expert on security policy at Masaryk University in the Czech city of Brno, said the Czech Republic wanted to “demonstrate its self-confidence and capability for resilience toward Russian aggressive behavior.” But he added that “the final effect strongly depends on support from Czech allies in the European Union and NATO.”