LONDON — In the early spring of 2014, the world watched, astonished, as soldiers without insignia took over government buildings in Ukraine’s Crimea peninsula, surrounded Ukrainian military bases and installed new leaders in the region. President Vladimir Putin of Russia claimed at the time that these were not Russian soldiers. Instead, he said, these were “local self-defense units.” Less than a month later, Russia annexed Crimea. And a year after that, Mr. Putin admitted what everyone had suspected all along: Yes, Russian soldiers had been involved.

That was when the term “plausible deniability” became a standard part of Western discussions about Russia. The practice of carrying out dubious deeds through the hands of proxies or other hard-to-identify agents has since become something of a trademark of Mr. Putin’s. Sometimes, the agents are indeed Kremlin actors in disguise, as they were in Crimea. But in the years since, Mr. Putin has pushed this tactic even further, implicitly or explicitly encouraging independent agents to act on their own, keeping the Kremlin’s hands clean. On many occasions, this has left the West a helpless bystander, unable to force Russia to account for its actions.

But time may be running out on this tactic. The attempted murder of Sergei Skripal, a former Russian double agent who was found unresponsive in southwest England earlier this month, poisoned with a deadly nerve agent, may be the moment when “plausible deniability” has reached its limits. In fact, it now looks as if it is turning against its masters in the Kremlin. The United States’ decision on Monday, alongside Canada and a number of European countries, to expel Russians in retaliation for the poisoning makes clear to Moscow that its actions have consequences, whether it denies them or not.

Since the annexation of Crimea, Russia has resorted to “plausible deniability” again and again. The interference in the American presidential elections was a classic case: Mr. Putin has repeatedly emphasized that Russia has not intervened “at the level of the government,” but he admits that some “patriotic hackers” or trolls with Russian citizenship might indeed have been active. The Russian president has also attempted to reap policy benefits from the denied action, steering the conversation toward his own priorities: accusing the United States of interfering in Russia and making the case for cooperating to regulate the internet and social media.