Five years ago, a Ukrainian graduate student in history, Mykola Balaban, came across Tony Judt’s monumental “Postwar: A History of Europe Since 1945.” The book, a breathtaking synthesis of all of Europe—art and war, people and ideas, East and West—in the second half of the twentieth century, is nine hundred and sixty pages long. (Its author, a longtime professor at New York University who never muted his sharp British irony, was a friend of mine.) “I read it almost in one breath,” Mykola told me.

Mykola was in the process of conceiving a dissertation proposal: a micro-history of Lviv between June 22nd and July 5th, 1941. These were the first two weeks following Nazi Germany’s attack on the Soviet Union; in Lviv, it was a time of anarchy between the retreat of the Red Army and the arrival of the Wehrmacht. Ordinary people looted shops; the N.K.V.D. shot prisoners; nationalists declared Ukrainian independence; Hitler's Einsatzgruppen executed university professors, mostly Poles; Ukrainians murdered Jews. This would not be a popular topic in Ukraine. Mykola was taken with the fearlessness of the author of “Postwar,” with Tony’s willingness to disregard academic fashion, “to go against the current.” Mykola also found in Tony’s work a way of understanding Eastern Europe in the context of Europe as a whole, a way of thinking that “precluded the orientalizing of oneself.”

Mykola completed part of his graduate studies in Poland, together with his friend Bohdan Solchanyk, another history student from Lviv. In the evenings, they would drink Spanish wine and eat French cheese in the kitchen of the University of Warsaw’s student dormitory. They talked about history: about the Soviet Union in comparison to other socialist countries, about the crimes of Stalinism, about the New Left. One night they came up with the idea of organizing a series of discussions about Tony Judt’s work.

Then came the Maidan. On November 21, 2013, the Ukrainian president, Viktor Yanukovych, unexpectedly declined to sign an association agreement with the European Union, a reversal of his own stated foreign policy. Around 8 P.M. on that day, a thirty-two-year-old Afghan-Ukrainian journalist, Mustafa Nayyem, posted a note on his Facebook page: “Come on, let’s get serious. Who is ready to go out to the Maidan”—Kiev’s central square—“by midnight tonight? ‘Likes’ don’t count_._”

As autumn ended and the winter began, the government’s violence against the protesters on the Maidan increased. Yanukovych’s riot police were using water cannons in sub-freezing temperatures. Protestors were disappearing, their bodies sometimes found frozen in the forest, signs of torture on the corpses. On Tuesday, February 18, 2014, Yanukovych’s riot police started to use live ammunition. Mykola, having spent two weeks on the Maidan, had just then returned home to Lviv to recuperate. In Lviv, he saw Bohdan, who said nothing to him about his intention to go to Kiev that night.

On Thursday, February 20th, another friend found Bohdan’s body amidst a pile of corpses in front of a McDonald’s. A government sniper had shot the young historian to death. The friend who found Bohdan’s body saw that Bohdan’s fiancée, Marichka, had called his cell phone seventeen times.

Three months after the massacre on the Maidan, many of Tony Judt’s friends and colleagues on both sides of the Atlantic went to Kiev to meet with one another and support the revolution that had begun with Mustafa Nayyem’s Facebook post. The effort reflected Tony’s insistence on the historian’s moral responsibility not only to understand, but also to engage.

The congress was the idea of Leon Wieseltier, whose friendship with Tony ended in 2003, when Tony published “Israel: The Alternative” in The New York Review of Books. In that essay, Tony made a merciless argument: it was contradictory to wish for both a Jewish state and a democracy. His anti-Zionism resembled the anti-communism of Arthur Koestler: a passion that comes from having once been on the inside. After the essay appeared, Leon did not speak to him for years. In August, 2010, Tony died of amyotrophic lateral sclerosis. If Tony were here, Leon told Timothy Snyder, my husband, as they were organizing the Kiev congress, he would have been the first person Leon would have asked to come.

In his opening address in Kiev, Tim evoked “the tradition of Tony Judt, the great historian of Europe of his era, who understood that the West made no sense without the East, and politics no sense without ideas.” Tony had come to ideas early, and Eastern Europe much later. Marxism, he once told me, had been the air he had breathed as a child from an Eastern European Jewish immigrant family growing up in postwar Britain. Later, as a Cambridge student, he was among thousands who gathered in Paris in May, 1968, “jump[ing] up and down quite so enthusiastically at the demonstrations as we shouted Ho, Ho, Ho Chi Minh.”

Only with time did Tony come to see that, in 1968, history was being made much less in Paris than it was in Warsaw and Prague. He came to understand this through friendships with several of the brightest among his Polish contemporaries, who, precisely at that moment when Tony was jumping up and down shouting “Ho, Ho, Ho Chi Minh,” were sitting in communist prison.

“Past Imperfect: French Intellectuals, 1944–1956 ,” an excoriating attack on Jean-Paul Sartre, Maurice Merleau-Ponty, and other French philosophers turned Stalinist apologists, is Tony’s settling of accounts with himself in that context. For a long time, Sartre was the closest thing European intellectuals had to God; from Tony’s point of view, this was why Sartre bore so much responsibility. Sartre’s “notable silence,” writes Tony, was his silence in the face of “the blood of others”—those tortured and executed during the postwar Stalinist terror in Eastern Europe. The West, Sartre and his friends believed, could not criticize communism because Western intellectuals were distorted by bourgeois capitalist thinking. “Nothing in Sartre’s other achievements comes close to me to compensating for his refusal to intervene or even speak out when faced with the show trials in central Europe,” Tony insisted to me, nearly two decades after he had written “Past Imperfect.”

“We are unwise to laugh too quickly at those who describe the world as a conflict between good and evil,” Tony said, in a lecture in 2003. “If you can’t use the word ‘evil,’ you have a real problem thinking about what happened in the world.” In February, 2014, the Polish philosopher Marcin Król told an interviewer that Europeans were facing a serious political crisis and a potentially fatal spiritual crisis: they had ceased asking themselves metaphysical questions, questions like “Where does evil come from?” As Król’s friend Adam Michnik, the Polish writer and dissident, once said to Václav Havel, “This is a civilization that needs metaphysics.”

The Maidan was the return of metaphysics. It was a precarious moment of moral clarity, an impassioned protest against rule by gangsters, against what in Russian is called proizvol: arbitrariness and tyranny. It united Russian-speakers and Ukrainian-speakers, workers and intellectuals, Ukrainians and Jews, parents and children, left and right. The Ukrainian historian Yaroslav Hrytsak, an adviser to both Mykola and Bohdan, described the Maidan as akin to Noah’s Ark: it took “two of every kind.” For Yaroslav, the wonder of the Maidan was the creation of a truly civic nation, the overcoming of preoccupations with identity in favor of thinking about values. People came to the Maidan to feel like human beings, Yaroslav explained. The feeling of solidarity, he said—it cannot be described.