When François Mitterrand visited China in 1961, Mao Zedong mocked reports of famine in the country. There was no famine, he said, only "a period of scarcity", an assertion that Mitterand - who described Mao as "a great scholar known in the entire world for the diversity of his genius" - was happy to accept. Returning to France after his three-week tour, Mitterrand had no doubts about his account of events: "I repeat in order to be clearly understood - there is no famine in China." Western politicians of the right shared the French Socialist leader's view. After touring China in late 1960, the Conservative MP for Chester, John Temple, reported that communism was working and that the country was making "great progress".

At the time these western dignitaries were making their escorted tours through China, it was in the grip of the largest famine in history, a man-made catastrophe in which at least 45 million people were starved, beaten, tortured or worked to death. Though Mao's Great Leap Forward was celebrated in the west as a major advance, the reality was captured by the name villagers gave to the vast irrigation schemes on which they were forced to labour - they called them "the killing fields". At least 2.5 million of the famine's victims died violently. Assaults against women were pervasive, including rape by party cadres and beatings for women who miscarried while being forced to work during the last stage of pregnancy. In some collectives, people were divided into groups and food distributed according to capacity for work - a type of "performance feeding" similar to that practised in the Nazi labour camps, with similar results (the old and infirm soon perished). In order to punish their families, those who died of beatings in rural communes might be left unburied; in some cases, their bodies were rendered into compost. Human flesh was traded on the black market, sometimes mixed with dog meat.

Once full of sound, the countryside became a place of silence, with no birds left in the trees, which had been stripped of their bark and leaves. Livestock was confiscated, often only to die of neglect. Children too weak even to cry were left to die in empty fields. Suicide was epidemic; between one and three million of those who perished during the Great Leap Forward took their own lives. Uncounted others vanished after being denied food in the communal canteens.

What Frank Dikötter describes as a "war on the people" was also a war against their environment. About 40 per cent of all housing was destroyed, sometimes so that the rubble could be used for construction or fertiliser, at other times in order to punish the occupants, who ended up destitute and homeless. China's landscape was also a target for attack. As Mao put it, "There is a new war: we must open fire on nature." The result of his campaign (which included the infamous "war on sparrows") was deforestation, soil erosion, landslides, floods, droughts and insect infestation. As Dikötter writes, "Mao lost his war against nature. The campaign backfired by breaking the delicate balance between humans and the environment, decimating human life as a result."

Using first-hand reports from party archives that have opened in the past few years, Mao's Great Famine is a masterpiece of historical investigation into one of the world's greatest crimes. Writing throughout in a sober and restrained style that only highlights the horror of the events it records, Dikötter shows in rigorous detail how responsibility for the disaster must be traced back directly to Mao. It was Mao who bullied his fellow communists into the Great Leap Forward, ordering purges of over three million "rightist elements" accused of questioning his policies. As late as the summer of 1959, a change of direction could have limited the casualties of the famine to millions. But Mao pressed on, and tens of millions died. Uncovering the magnitude of this terrible crime, Dikötter has produced one of the few books that anyone who wants to understand the 20th century simply must read.

Limiting himself to describing and analysing the famine, he devotes only a few lines to what may be one of its eeriest aspects: that it provoked so little reaction in the west. It is not that the fact of the famine was unknown. Reports surfaced repeatedly, only to be discounted by a host of prominent visitors. As Jasper Becker recounts in Hungry Ghosts: China's Secret Famine (1996), a pioneering study cited by Dikötter, Mitterrand was by no means the only western dignitary who heaped praise on Mao's China as a society where chronic hunger no longer existed. There were also the Argentinian revolutionary Che Guevara, the Swedish sociologist Gunnar Myrdal, the art critic and self-styled anarchist Sir Herbert Read, the distinguished Cambridge Sinologist Joseph Needham, the American liberal economist J K Galbraith and Graham Greene's cousin Felix, a some-time New Age guru who was a shameless apologist for the regime, along with many others.

Not everyone joined in the chorus of denial. Testimony from refugees was accepted by a number of western opinion-formers, including Bertrand Russell, while the Guardian and the New Statesman urged the US to send food aid. The Red Cross offered help, which Beijing refused because the organisation had had the temerity to inquire about conditions in Tibet. But these were exceptions, and for the most part the catastrophe was ignored.

Why the famine failed to provoke a larger response is unclear, but one reason may be that western opinion tends to be friendly to oppression so long as it serves utopian dreams. History records many famines that were largely or wholly man-made, such as those in 18th- and 19th-century Ireland and British India. The famine Mao made was different, not only in that it was bigger, but because it was created by a regime that claimed to be creating a society without scarcity. As Dikötter notes, "A vision of promised abundance motivated one of the most deadly mass killings in history." As in the Soviet Union, where Stalin was able to engineer the vast Ukrainian famine without stirring practically any criticism in the west, an imaginary paradise coexisted with the reality of rampant death.

In the minds of its western admirers, Mao's China was a fantasy land, not a real country. Viewed from the safe vantage point of affluent boredom, the spectacle of revolution seems to generate a voyeuristic excitement not unlike that provided by media images of celebrity death. Did the western dignitaries who toured the killing fields really believe that nothing was being hidden from them? Or were they secretly thrilled to be privileged spectators at one of history's greatest experiments in terror? These are questions that cannot be answered, but it may be worth reflecting that China has never aroused more enthusiasm in the west than during the Cultural Revolution, another catastrophic experiment, set in motion by Mao only a few years after the Great Leap Forward, which destroyed millions more lives.

John Gray is the New Statesman's lead reviewer. His latest book is "Gray's Anatomy: Selected Writings" (Penguin, £10.99)