News update: On Friday, Russian state media said that Aleksei Navalny has died in prison.

In 2003, four years into President Vladimir Putin’s tenure and with liberal politics at a nadir, I visited the Moscow office of the center-left Yabloko Party. There, among the 50-something inveterate anti-communists who dominated the party, sat a 27-year-old staff member. It was Aleksei Navalny.

We quickly became friendly. We were members of the same generation and the same culture, we had the same views about good and evil, and we spoke the same language. Though we didn’t always agree, I felt that he and I were — to quote Rudyard Kipling — “of one blood.”

It was obvious that such a man would feel cramped in a stuffy old party, and so it turned out. But it was not at all obvious that Mr. Navalny would rise to be the undisputed leader of the opposition, his challenge to Mr. Putin’s rule so profound that it would lead to the regime’s efforts to silence him — through attempted assassination, imprisonment and illness that last week threatened to tip over into death — and to thousands of Russians, across the country, taking to the streets to protest his mistreatment. Russia now has two national leaders: Mr. Putin in the Kremlin and Mr. Navalny in prison.

When we first met, such a future — if someone had beamed back from 2021 to tell us of it — would have seemed far-fetched. At the time, an opposition movement was slowly taking shape. And at comings together of opposition figures, united in their disagreement with Mr. Putin if little else, I would often see my old acquaintance Aleksei Navalny. He understood that real political life was there, not in his own party, and he rapidly became one of the notable figures of the opposition.

His expulsion from Yabloko in 2007 for participating in a nationalist demonstration — which he’d attended out of solidarity with the nationalists facing the Kremlin’s repression — was no great loss for him. He had already created a name for himself as the author of a popular blog: While others cursed Mr. Putin and held protests for freedom of assembly, Mr. Navalny set to work exposing abuses in state corporations and accusing the authorities of thievery. Criticizing corruption was more convincing than slogans advocating democracy.

Yet the turning point, for the country and for Mr. Navalny, came in 2011, when Mr. Putin decided to become president of Russia again and his party, United Russia, gathered a wildly improbable majority. Mass protests broke out, and the leaders of the opposition, who had grown accustomed to seeing a hundred or so people at their gatherings, suddenly found themselves looking out at tens of thousands of citizens.