HE RAN the country for less than four years, yet between April 1975 and January 1979 Pol Pot killed up to a fifth—some think a quarter—of the Cambodian people to whom he said he was bringing a new and better life. In its way, it was the worst of the 20th century's totalitarian horrors, unless the eventual unlocking of North Korea's doors reveals something even grimmer. Hitler murdered about 6m Jews and others in his concentration camps; Stalin's “anti-party” toll was close to 20m; Chairman Mao's Great Leap Forward starved over 20m to death, before moving on to the Cultural Revolution. But Pol Pot's victims were a much bigger proportion of little Cambodia's 7m people, and few of them could even vaguely be called “enemies of the regime”. His killing fields were the most mind-boggling of them all.

Philip Short, who wrote a good book about Mao's China, has now done a spectacularly efficient job of describing what happened, and how. He has spent four years in Cambodia, talking to survivors of the killing-fields, and perpetrators. He has dug up piles of revealing documents. Some of the brightest illumination comes from the handful of westerners who watched what was going on, not least the diaries of Laurence Picq, an honest young Frenchwoman who went to Cambodia thinking she could help a good cause.

The result is a chillingly clear portrait of Saloth Sar, the man who became Pol Pot (and also Grand Uncle, First Brother and sundry other pseudonyms). From a comfortable background—his sister was one of the king's concubines—he went to a smart lycée in still French-run Phnom Penh, and then won a scholarship to study in Paris. There he fell in love with Marxism-Leninism in its especially intellectual French form, and from France he went back to the emerging guerrilla war in Cambodia, to bring communism to his countrymen. Calmly and firmly, he worked his way to the top of the party; and in April 1975 his men marched into Phnom Penh.

It then became pure Orwell. Pol Pot at once ordered the total evacuation of all towns and cities—not just the middle class, but labourers, mechanics, street-cleaners, war refugees, everybody. All Cambodians were to become workers on the land. There were to be no wages. Meals were to be provided by collective kitchens (“unity of feeding”). Each Cambodian had to refer to himself or herself as “we”, forbidden to use the first person singular. When one region found it did not have enough food, supplies were not sent from better-off places; rather, the hungry were marched off to look for them.

Of course, it did not work. Up to 1m people died of starvation. Protests began, including among party members. The leadership of the party denounced such “microbes”. The protesters were put into camps, including Camp S-21 at Tuol Sleng, the sole task of which was to extract confessions. Many “confessions” turned out to be pure invention, yet all confessors were executed. At least another 100,000 people, maybe 250,000, died at this stage of the proceedings. As Pol Pot's central committee put it, it was necessary “to avoid a solution of peaceful evolution”, which could “corrode” the revolution.

Why was Pol Pot's Cambodia even worse than Mao's China, Stalin's Russia and Hitler's Germany? Here Mr Short, so good at finding out what happened, is less good at explaining why it did.

He suggests that Pol Pot, like many other Cambodians, was driven by resentment over his country's loss of glory since the great days of the Angkor empire. But that was 600 years earlier. Lots of other countries have had far more recent puncturings of national pride without being pushed into anything quite as horrible.

Mr Short then wonders whether Buddhism, Cambodia's main religion, lay near the root cause, because it believes in “the demolition of the individual”. This is nonsense. Buddhism, a gentle faith, believes that individual human beings eventually dissolve into nirvana when in successive lives they have earned it. This is not the explanation of Pol Pot's slaughters.

No, it was the bug he picked up in Paris that poisoned Pol Pot. An ideology which believes, as communism did, that a small group of self-selected possessors of the truth will get everything right is bound to produce disaster. Perhaps things were made worse by Pol Pot's desire to outshine the communists in Vietnam; and maybe also by some still unexamined twist in his psyche. All the same, it was the pseudoscientific certainty of Marxism-Leninism, that malformed child of the Enlightenment, which was chiefly to blame.