But Putin also needs to beware. Dictatorships fall not only when they have implacable opponents but also exemplary victims: Steve Biko in South Africa, Benigno Aquino in the Philippines, Jerzy Popieluszko in Poland. Through their deaths, they awakened the living to the conviction that it was the regime that should die instead.

Today, Nemtsov continues to haunt the Kremlin. So do Sergei Magnitsky, Natalia Estemirova, Alexander Litvinenko and Anna Politkovskaya, to name just a few of the regime’s murdered adversaries. At some point, a growing list of victims will start to weigh heavily against Putin’s chances of staying in power. The death of a galvanizing opposition figure could be the tipping point.

Especially when the political-survival formula that has worked for Putin so far is coming unstuck. That formula — enrich your cronies, terrify your foes, placate the urban bourgeoisie with a decent standard of living, and propagandize everyone else with heavy doses of xenophobic nationalism — no longer works so well in an era of Magnitsky sanctions, international ostracism, a persistently stagnant economy, middling oil prices, unpopular pension reforms, and dubious foreign adventures.

It works even less well when your domestic foes aren’t so easily terrified. As in Hong Kong, a striking feature of the Russian protests is the extent to which they are youth-driven — a vote of no-confidence in whatever the regime is supposed to offer. One recent survey found that the number of young Russians who “fully trust” Putin fell to 19 percent this year, from 30 percent last year. That’s not a good trend line for a man who aspires to die on his throne.

None of this guarantees that Putin can’t bounce back, not least if Donald Trump gives him the kinds of breaks, like readmission into the G7, he needs . And Robert Mugabe’s death this week at 95 is a reminder that tyrants can endure longer than anyone expects.

Still, for the first time in 20 years, the elements by which Putin falls are coming into place. Core among them is the courage of people like Sobol — a woman who, as Pericles said more than 2,400 years ago, “knows the meaning of what is sweet in life and of what is terrible, and then goes out undeterred to meet what is to come.”