To understand the moral and literary power with which Robert Conquest wrote, consider the second sentence in his book Harvest of Sorrow: Soviet Collectivization and the Terror-Famine, a study of the 14.5 million deaths that resulted from Joseph Stalin's murderous takeover of his nation's agricultural sector: "We may perhaps put this in perspective in the present case by saying that in the actions here recorded about twenty human lives were lost for, not every word, but every letter, in this book."

As Conquest's friend, the British novelist Martin Amis, would later observe with a palpable shudder, "The sentence represents 3,040 lives. The book is 411 pages long." The math is too terrible to contemplate.

Conquest, who died of pneumonia on August 3 at the age of 98, was many things: a highly regarded anti-modernist poet, a military intelligence officer, a diplomat, a scholar, a ribald prankster, a serial non-monogamist (four wives punctuated with countless entanglements), even a dystopian science fiction novelist.

But most of all he was Stalin's personal prosecutor, over and over placing him in the dock of history to answer for his monstrous crimes. Conquest wrote more than a score of books on Soviet history and politics, two of them—Harvest of Sorrow and 1968's groundbreaking The Great Terror: Stalin's Purges of the Thirties—considered the definitive texts on the pure wickedness of the events they describe. "I know that after my death a pile of rubbish will be heaped on my grave, but the wind of History will sooner or later sweep it away without mercy," Stalin once said. Fortunately, Conquest was there to stack it right back up.

The British-born son of an American father and an English mother, Conquest was educated at Oxford, where he joined the Communist Party and visited the Soviet Union in 1937. That started a process of disillusionment that gained speed when Conquest served as a British military intelligence liaison to Russian-commanded Bulgarian resistance forces and hit critical mass when he stayed on in Bulgaria as a diplomat after the war and witnessed Stalin's brutal Sovietization of the country. By 1948 he was back in London, writing an increasingly hostile series of Foreign Office research papers on Soviet activities in Eastern Europe that would eventually morph into his books.

The first of those, Common Sense About Russia, appeared in 1961. But it wasn't until 1968, with the publication of The Great Terror, that Conquest truly hit his stride. Drawn from emigre memoirs, dissident samizdat documents, and sworn statements by hundreds of Soviet exiles and defectors who testified in a 1948 libel trial against a French Communist newspaper, The Great Terror was the first systematic compilation of the atrocities committed during Stalin's massive purges of 1936 through 1938.

The purges themselves were hardly news, but the world had mostly fixated on the manifest injustice of the Moscow show trials of a relatively small group of disgraced top officials charged with spying and sabotage. Conquest forced attention to the massive body count—a million or more—among ordinary citizens, who were shot in prison basements or sent to starve to death in Arctic work camps.

But The Great Terror was much more than a morgue census. Conquest's compelling eye for detail, coupled with a stark, understated prose style, combined to produce the greatest horror story of the 20th century.

He wrote of a town in Byelorussia where a group of peasants stumbled into what may have been the perpetually depressed Soviet economy's single growth industry: professional informing. They routinely partied after trials with the 15 rubles a head they were paid to denounce neighbors as spies, hoarders, and "wreckers," as saboteurs were known. They even wrote an epic ballad about some of their most successful denunciations.

He wrote of the urkas, the labor-camp gangs of common criminals so violent and depraved that even the guards feared them and refused to make them work. The hideously tattooed members, sporting names like Hitler or The Louse, instead spent their days plotting mass rapes of female inmates and gambling for the clothing of newly arrived political prisoners; the losers had to strip it from the victims and deliver it to the winners.

He wrote of Stalin's workdays, which usually began by leafing through hundreds of secret-police-recommended death sentences left in his morning inbox, perhaps with the help of his sycophantic adviser Vyacheslav Molotov. December 12, 1937, was a typical day, Conquest reported: "Stalin and Molotov sanctioned 3,167 death sentences, and then went to the cinema."

Not that being a bloodthirsty dictator was all work and no play. Conquest described Stalin laughing until he cried as an executioner acted out the final, sobbing moments of his former crony Grigory Zinoviev. "Stalin was overcome with merriment and had to sign to [the performer] to stop," Conquest wrote.

"The Great Terror is an extraordinary book, and even more extraordinary is that he extracted it from a totally closed society," says the Emery historian Harvey Klehr, who has written extensively on Soviet espionage in the United States. "The information was out there, but nobody had thought of collecting it using those sources."

As skeptics of the Cold War gained the upper hand in American academia, Conquest's work was dismissed as reactionary fantasy and criminal libel. But in the years after the collapse of the Soviet Union, as Moscow's archives began dribbling out to the public, his reporting was confirmed and judged by some even a bit too mild.

Conquest was, of course, gratified. Martin Amis reported that, when a publisher asked for a new title for a revised edition of The Great Terror, Conquest suggested: "How about I Told You So, You Fucking Fools?" The historian later said, not altogether convincingly, that the quote was the jocose fabrication of a friend. But there is little doubt that he considered most of his critics fools, and fools who needn't be taken seriously.

In 1988, The Village Voice published a shrill attack on Harvest of Sorrow that accused Conquest of, among many other things, "red-baiting" Walter Duranty, the New York Times correspondent in Moscow whose Panglossian reporting on Stalin won him a now-discredited Pulitzer Prize. Among other things, Duranty's stories repeatedly denied any famine in the Soviet Union during 1932–33, though we now know somewhere between 6 million and 8 million peasants starved to death at the time. Pshaw, sniffed Conquest. "Duranty wasn't a red at all, just a self-serving liar," he wrote the Voice. "I think liars should be baited. Dupes, too, perhaps less harshly."