Arthur Koestler, that one-time titan in the Cold and culture wars, was born 100 years ago this year. Those who remember him tend to think of Koestler (in more or less descending order of interest) as a high journalist of near genius; a battle-hardened and even invincible polemicist; a scientific popularizer of something well short of genius; a dubious dabbler in the occult. But that is not all. If it were, his centenary would be little more than an occasion to muse on fleeting fame. He was also a novelist, and, in “Darkness at Noon” (1940), his masterpiece about the Soviet Terror, Koestler wrote one lasting book – just one, but one is enough – that once upon a time really did help to change the world.

The man was hounded by controversy in life, and it hasn’t abated in death. Yet clarity is on the way. Michael Scammell’s biography, long awaited, will appear next year, and is likely to supersede David Cesarini’s hostile, hasty, and rather contorted biography of 2000. Mr. Scammell almost defines judiciousness. Count on it, no contested stone will be left unturned. A proponent of euthanasia, Koestler killed himself in 1983 at the age of 78. Enfeebled by Parkinson’s disease and leukemia, he called off a losing battle by swallowing a carefully hoarded cocktail of lethal drugs. The move seemed eminently rational. But was it? Koestler’s younger, perfectly healthy, and very long-suffering wife, Cynthia Jeffries, killed herself at his side – flinging herself, so to speak, on the funeral pyre of his reason – in what many felt may have been only the last and most outrageous of her many acts of irrational self-sacrifice.

Cynthia Jeffries was not alone in loving him, but Koestler was anything but a lovable man. Truman Capote, who used to glimpse him in Paris, thought Koestler looked like a dwarf locked in a permanent rage, forever just about to throw a punch. His relations with women were exploitative and rapacious. He knocked back alcohol in quantities that made him often abusive, sometimes violent, and, behind the wheel, a real menace. Koestler’s best journalism was crystalline and erudite, capped with charm and humor, as when in 1973 he defined Bobby Fischer, the lunatic chess champion, as a “mimophant”: a creature sensitive as a mimosa about his own feelings, and equipped with an elephant’s hide about everyone else’s. In fact, one mimophant had spotted another. In a good mood, Koestler could be suspicious and testy; in a bad one, intolerable. With enemies, he was lethal; with friends and allies, though capable of real kindness and generosity, he was pugnacious, self-righteous, and blind with arrogance. He spent a lifetime lurching after self-knowledge. He never quite caught it.

Falling into a classic trap of the multitalented, Koestler rather mistook the true nature of his own gift. After “Darkness at Noon” made him a world figure, this born polemicist dreamt about being a great mind, counted in the company of Freud and Mann and von Neumann. A grave mistake. For one thing, Koestler’s mind, while perfectly good, was nowhere nearly that good. For another, his high ambitions were undermined by his character flaws. His forays into the history of science and such metaphysical issues as the mind-body problem invariably became entangled in grandiosity and compromised by his unconquerable impulse to pick a fight. Koestler was an intellectual street fighter, not an Olympian. Trained as a journalist and propagandist, he transcended them only by lifting polemic to the level of art.

Yet art it is. “Darkness at Noon” stands not just as a great indictment of the Soviet Terror but as an enduring vision of the diseases of reason. Rubashov, a sometime hero of the Russian Revolution, finds himself a defendant in the purge trials of the late 1930s. The trials are obvious frauds: Rubashov has done nothing wrong. Yet he is doomed: All hope abandon, ye who enter here. Mingling torture with “rational” persuasion, the secret police set out to make Rubashov “confess” to a criminalized revision of his whole existence, condemning everything he has served and believed. They succeed. Rubashov submits and bows before the minions of the lie. His thoughts, he admits, were not always innocent. History has made him “objectively” guilty; if the Revolution must be right, Rubashov must be wrong. It is the triumph of the collective lie over individual truth. Their debate is an insane quarrel about the moral authority of terror and the merits of murder as a higher form of humanism. Rubashov and his killers dance around each other like mongoose and cobra. It is the insidious intellectual passion of their exchange that makes the book so compelling. “Darkness at Noon” is a novel of ideas, but a fierce one, sweeping us to its murderous conclusion with cold, ardently reasoned fury.

It puts the young Koestler’s own beliefs on trial. He conceived the book at the age of 32, in 1938. By then he was already a somebody. He’d been a star journalist in Weimar. In the German Communist Party, he’d become a favored protege of Willi Munzenberg, that “red eminence” who was a founding father of the culture wars, the Comintern’s great propaganda master in Europe.

During the late 1930s, as the Great Terror consumed communism worldwide and Stalin inched toward his alliance with Hitler, Koestler began to back away. In the spring of 1939, with Europe on the brink, the incipient renegade went with his new British girlfriend, Daphne Hardy, to a shabby villa in the south of France, where he began to write the novel. He was hard at work when the Nazi-Soviet Pact precipitated World War II. Koestler hurried back to Paris, still reasoning his way, page by page, toward Rubashov’s suicidal submission – working until the French government packed him off to a concentration camp for enemy aliens at Le Vernet.

After a campaign for his release got him back to Paris – and Daphne Hardy – he worked non-stop, barely escaping a second internment, and finished the book. Then France fell. Daphne Hardy managed to spirit her English translation – the only duplicate typescript – across the channel. Koestler’s own escape reads like the “Casablanca” without Ingrid Bergman. Forging a false identity, he joined, and then fled, the French Foreign Legion. He finagled false papers. He made his way to the Spanish border; then to Oran; then to Casablanca; then to Lisbon.

When he got to England in November, the Battle of Britain was in full force; the Communists and Nazis were allies; civilization stood on the brink. Koestler was instantly interned – yet again – as an enemy alien. Another campaign got him out, but meanwhile Daphne Hardy had arranged for “Darkness at Noon” to be published by Cape. Koestler walked out of his British prison into fame.

Nightmares fade when morning comes. In the post-Soviet era, it is easy to forget that there was a time when the victory of mass murder and the Big Lie really did look like the future. “Darkness at Noon” was not the first challenge to Stalinism to emerge from the left, but it was among the most potent. Only Orwell’s “1984” surpassed it in impact. Translated into more languages than I can count, the novel was read by millions, and it changed millions of minds.

Today, the literature of anti-communism is unlikely to be read with the old urgency, much less by millions. Yet “Darkness at Noon” remains a great novel. As time shapes the shelf of lasting Cold War literature, “Darkness at Noon” will have its place, as will works once seen as beneath it, like John le Carre’s wonderful thriller about the Berlin Wall, “The Spy Who Came In From the Cold” (1963), or work emerging from obscurity, like Victor Serge’s “Case of Comrade Tulayev” (1948), recently reissued by New York Review Books, with a stunning introduction by Susan Sontag – her last essay, and one of her best.

And then there is the art to come. A new vision of the Cold War is on the way. “Cold War”: Those two words will not always be an epithet. Dickens’s “A Tale of Two Cities,” with its immeasurably influential picture of the French Revolution – inspired by Carlyle’s history – was written 70 years after the Paris terror. With the new and truer history of what happened between 1917 and 1989 appearing from historians, a new literature will follow. Only that – and time – can sweep away the wreckage of the old rationalizations and the old lies.

“Darkness at Noon” is an intellectual’s masterpiece. “Reason run amuck” is its theme. Few works of art have ever shown revolution’s horror, or savored its ironies, more bitterly. The book is so entangled in its own argument with evil that Koestler once received a letter from a young Frenchman, thanking him for converting him to Stalinism and its true light. “Darkness at Noon” will always be read by those struggling to understand: why? Why was half of humanity once subjected to the inhuman horrors of the police-state, trapped in the tyranny of a “rationality” that left them impoverished, terrorized, and reduced to material, moral, intellectual, and spiritual squalor – and every five or 10 years or so, mass murdered by the millions? Yes, indeed, that “rationality” is facing some mighty tough questions, one of which is why so many good and decent people had such faith in “reason run amuck.”

Propaganda, polemic, and prophecy: They are three antithetical yet interdependent voices. They mingle and merge in ways that madden. Koestler began his career as a propagandist, and the shoe fit all too well. His portrait of Munzenberg the propaganda king is wonderful: “‘Don’t argue with them,’ he kept repeating … ‘make them stink in the nose of the world … That, Arturo, is propaganda!'” Polemic is really propaganda transformed by a road-to-Damascus conversion to truth telling. It rises out of propaganda’s pit of lies because, despite the distortions of one-sidedness, polemic is a genuine vehicle of passion and truth that can and must be judged as such. Thus judged, “Darkness at Noon” will live as one of the great polemical works of modern literature.

Yet it is not quite a work of the very first rank. Just as polemic rises from, and transcends, propaganda; so prophecy rises from, and vastly transcends, polemic. Koestler was never able to achieve that transcendence. The harsh lonely heights of that highest peak on the range – the prophetic utterance that is the ultimate rebuke of truth to power – were beyond him. Maybe he was too reasonable. Prophets are not reasonable people – “Don’t argue with them, Arturo.” Prophets stand alone; they have nothing to lose; they are swept into the whirlwind. The sheer scale of totalitarian evil in the 20th century demanded something beyond mere argument: beyond reason, conviction, sanity, and decency. It called for all-consuming prophetic rage. That is the force that drove some very great hearted Russians – Nadezhda Mandelstam, Evgenia Ginzburg – beyond despair to the big hill itself. Solzhenitsyn very nearly made it to the top.

Arthur Koestler never achieved stature like theirs. It’s true, he often gazed longingly at the great peak, and we sometimes shiver as its majestic shadow sweeps, briefly, over some page in his work. But then it is gone. The sacred mountain itself was not his to climb.

Mr. Koch is the author of “The Breaking Point: Hemingway, Dos Passos, and the Murder of Jose Robles,” just out from Counterpoint, and “Double Lives: Stalin, Willi Munzenberg and the Seduction of the Intellectuals.”