BRUSSELS — When leaders of the European Union gathered last Thursday night for a working dinner inside their lavish new headquarters, the conversation turned to Russia. Over scallops and lamb, Prime Minister Theresa May of Britain shared intelligence detailing the high probability that the Russian state had carried out the poisoning of a former Russian spy on British soil.
Usually, Mrs. May is the odd woman out at European Union gatherings, given that she is trying to negotiate Britain’s departure from the bloc. But not this time.
By the next morning, European Union leaders agreed that a coordinated response was needed, according to four senior European officials. Then President Emmanuel Macron of France said everyone should go home and consider expelling Russian diplomats.
“Let do it on Monday at 3 p.m.,” the French president said, according to one of the senior officials, all of whom asked for anonymity given the sensitivity of the issue.
The European Union is not usually a model of decisiveness, but the expulsion of Russian diplomats across the Continent on Monday was a dramatic and pointed gesture. It came in concert with a similar, larger move by the United States, which expelled 60 Russians, and signaled a new, tougher effort to punish bad behavior by President Vladimir V. Putin of Russia.
“I can’t think of any previous occasion when so many countries have coordinated on expulsions,” said Ian Bond, a former British diplomat in Moscow, adding that for many of the smaller countries, “it’s the first time since the Cold War that they’ve even expelled one Russian diplomat.”
Russia is always a tricky issue for the European Union, given its critical role as an energy supplier to the Continent, as well as the divided opinion among leaders on how confrontational, or not, the bloc should be with Mr. Putin.
But the March 4 poisoning in Salisbury, England, of the former Russian spy, Sergei V. Skripal, and his daughter, Yulia, crossed a line. British authorities say they were exposed to the nerve agent Novichok, marking the first use of a chemical agent on European soil since before the Second World War.
The brazen nature of the act was too much for European officials to ignore.
“This is an intelligence operation carried out with intelligence capacity with weaponized, weapons-grade chemical agents,” one senior European official said. “It has taken matters to an entirely different level.”
Alluding to Russia’s earlier aggressions in Ukraine, the senior official added: “Russia keeps violating international law in Crimea and Ukraine and unwritten rules on nonintervention, and now there is the use of nerve agents in Britain.”
In Britain, Mrs. May had already expelled 23 Russian diplomats earlier this month, while members of her cabinet spoke in increasingly strident tones against Mr. Putin. Her remarks last Thursday night seemed to stiffen the spines of other European leaders.
Mr. Macron and Chancellor Angela Merkel of Germany were prominent supporters of Mrs. May’s call for action, having planned tactics with Britain before the dinner. The French had provided the British with technical assistance on analyzing the poisoning case and come to the same conclusion. And when the Franco-German couple agree, others tend to fall into line, even if grumpily.
The decision was finalized Monday morning, as European Union ambassadors met in Brussels to describe what each country was prepared to do. A statement was prepared for Donald Tusk, the president of the European Council, at a meeting in Bulgaria, and the result was extraordinary — 16 European Union countries had agreed to expel one or more Russian diplomats, and others, like Ireland, were considering joining in.
Europeans have gotten used to the fact that between one-third and one-half of all the Russians at Western embassies, the European Union and NATO are working in intelligence, the official said. But Russia has now made that impossible to ignore.
There was satisfaction in Brussels over the outcome, with even Hungary, which has warm relations with Mr. Putin, agreeing to expel a Russian diplomat. Greece and Cyprus, with close ties to Moscow, were unwilling to do so without a smoking gun, and some small countries, like Malta, did not want to lose all representation in Moscow and risk breaking relations entirely.
Austria was disappointing to some, refusing to expel anyone now that the far-right Freedom Party controls the Interior Ministry.
Bulgaria, which is currently holding the bloc’s rotating presidency, begged off, citing the need for neutrality, though its ties to Moscow are clear.
“We all back Britain’s position,” Prime Minister Boiko Borisov of Bulgaria said on Friday. “While there is high likelihood, but no evidence, we cannot decide on the matter.”
The Czech Republic, which expelled three Russians, was a particularly interesting case, because the messaging was divided, even as Russian media outlets had suggested that the nerve agent could have come through there.
Acting Prime Minister Andrej Babis said on Monday, “If our ally is in a serious situation and asks for help, we should come forward,” he said on Monday.
“Russia has crossed all limits when it declared that the poisonous substance Novichok might have come from the Czech Republic,” he added. “It is a complete lie and we strongly deny it.”
But the country’s more pro-Russian president, Milos Zeman, opposed the expulsions. In a statement, he called on the country’s intelligence services to examine if Novichok was ever made or stored in the country — even though government officials have denied it.
The Italian reaction troubled some, given the negotiations going on for a new government after the strong showing of populist parties in the recent election. Matteo Salvini, the leader of the League party, is an ally of Mr. Putin and criticized the expulsions.
“To boycott Russia, renew the sanctions and expel its diplomats doesn’t resolve problems, it aggravates them,” he wrote on Monday after Italy announced it would expel two Russian diplomats. “Dialogue is better. I want a government that works for a future of peace, growth and security. Am I asking too much?”
In Brussels, some officials said the coordinated expulsions proved that European solidarity can transcend even Britain’s decision to leave the European Union, known as Brexit, or the acrimonious negotiations over that decision. Others disagreed, yet Monday’s expulsions were clearly a win for Mrs. May.
In her statement to the House of Commons on Monday, Mrs. May said she had argued to European colleagues “that there should be a reappraisal of how our collective efforts can best tackle the challenge that Russia poses following President Putin’s re-election.”
She singled out France and Germany, adding: “In my discussions with President Macron and Chancellor Merkel, as well as other leaders, we agreed on the importance of sending a strong European message in response to Russia’s actions — not just out of solidarity with the U.K. but recognizing the threat posed to the national security of all E.U. countries.”