On Thursday, it was Americans who learned they were being kicked out of Russia. On Friday, it was Europeans. Russia’s Foreign Ministry summoned the ambassadors of several European nations and ordered the expulsion of their diplomats; their number precisely mirrored the number of Russian diplomats expelled by Western nations on Monday. In all, 28 nations expelled 153 Russians over the past week in response to Moscow’s alleged role in the attempted assassination by nerve agent of Sergei Skripal, a former Russian spy, and his daughter, Yulia, in the U.K. The U.S. expelled by far the highest number of Russians, 60, and Moscow responded in kind. The U.K. expelled 23 and faced similar retaliation.

But on Friday, Moscow went one step further: It ordered a reduction in the size of the British diplomatic mission in Russia to match the numbers in Russia’s mission to the U.K. It did not say what those numbers were.

Relations between Russia and the West are the “worst that we’ve seen since the Cold War, potentially worse than some periods of the Cold War,” Alina Polyakova, a fellow at the Brookings Institution in Washington, said.

Polyakova said that during the Cold War years, there were avenues of dialogue between the Soviet Union and the U.S., including military-to-military relations and “a certain set of rules that both sides knew to follow.”

“Now those kinds of rules aren’t so clear anymore. We don’t have the same kind of institutional structure for a bipolar world,” she said. “And for those reasons, it’s difficult to see where this downward spiral ends.”

Until Friday, Russia’s response to the expulsions of its diplomats was a precise tit-for-tat, suggesting that Moscow, which denies it had a role in the assassination attempt against the Skripals, didn’t want to make the situation deteriorate further by one-upping other countries’ reprisals. But the Russian Foreign Ministry’s order of the reduction in the size of the British diplomatic mission suggests there may be more actions ahead—unless the U.K. decides to accept the decision and not respond in a commensurate manner.

Moscow acted similarly last fall when it ordered the U.S. to reduce its diplomatic staff in the country by 755 people to match the number of Russian staff in the U.S. That move was a response to the Obama administration’s decision in December 2016 to expel 30 Russian diplomats from the U.S. and seize Russian diplomatic compounds in New York and Maryland—a move that came in response to Moscow’s interference in the U.S. presidential election.

“I don’t think this is a decision that the United States is taking lightly,” Nina Jankowicz, a global fellow at the Wilson Center, told me about the most recent round of expulsions. “I don’t think that they want to continue to do this. But Russia leaves us no choice with its behavior.”

Tensions between the two sides predate both the assassination attempt against the Skripals and the U.S. presidential election. They began after Russia’s annexation in 2013 of Ukraine’s Crimea region. Western nations imposed sanctions against Russia for its actions, which continue as it supports a breakaway region in Ukraine. The two sides are also at odds over the future of Syria where Moscow backs Bashar al-Assad; the future of the Iranian nuclear deal, which Russia, a signatory, supports (European countries do, too; the Trump administration has signaled it may withdraw from the deal); and other global conflicts. Indeed it is conflicts such as these that may be major sites of future tensions between the U.S. and Russia. Last month in Syria, for example, U.S. forces killed about 200 Russian mercenaries fighting on Assad’s behalf in Syria.

“More of this is going to continue to happen because the avenues of communication are not as good as they may have been during the Cold War period for deconfliction,” Polyakova told me.

There is another difference between the Cold War period and today, Jankowicz said: the differing nature of the administrations in Washington.

“In the Reagan administration, it was clear from every single official—from a random official at the State Department on up to President Reagan himself— where the Reagan administration stood on Russia. And to me it’s not really clear where the Trump administration stands.” She cited President Trump’s failure to publicly criticize Russian President Vladimir Putin, Trump’s assertion last November that Russia “can greatly help” on a host of issues from North Korea to Ukraine, and his reported resistance to selling arms to Ukraine. “There’s just such an incongruity in the decisions his administration is making and what the president says or does not say,” she said. “His silence to me is deafening—and that’s the difference between Reagan-era Cold War policies and what we’re seeing today.”

Ultimately, though, the tensions are unlikely to wane unless Russia changes course. The alleged use of a nerve agent in the U.K. is only the latest in a series of such alleged actions in the U.K. by Russia—actions that previously have mostly been met with silence from Western capitals.

“What the Russians have been doing is slowly testing the West, playing on the gray zone of the rules, see what the West will do, and now there’s been a response,” Polyakova said. “I think the ball is very much in the Russian court to decide: Are we going to continue this confrontation strategy of trying to test the West, or are we going to scale it back?”