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The British never really warmed to Hirohito, the bespectacled, bow-legged 124th Emperor of Japan.

"Nasty Nip in the Air" jeered Private Eye when he visited London in November 1971, adding "Piss Off, Bandyknees!", in case anyone missed the pun.

Students turned their backs as Hirohito drove through London, former POWs called for a boycott of Japanese products. "We cannot pretend that the past did not exist. We cannot pretend that relations between our two peoples have always been peaceful and friendly," the Queen explained.

We all knew what HM meant: the scholarly amateur marine biologist beside her had, 26 years before, been the war-lord on a white horse, worshipped as a living god, in whose name Japanese armies had ravaged China, overrun British colonies, brutalised British prisoners and effectively ended the British empire in the Far East (soon to be followed by the rest), as well as causing the violent deaths of millions of his own Japanese subjects. The scholarly Hirohito had quite a past to live down.

What sort of man was he? A quick-change artist? A hypocrite? Or, as he was presented - a reformed character? Herbert P Bix, an American professor at a Tokyo university, has no doubts - Hirohito was a thoroughly nasty piece of work, the "secret energiser", he charges, of Japan's war against China, which became Japan's war against the world.

Bix has one enormous, long-obscured truth going for his thesis: when the American conquerors of Japan decided not to put Hirohito on trial as a war criminal, originally out of military expediency, later as a Cold War ploy, but went ahead to hang his prime minister, General Hideki Tojo, and seven of his wartime cabinet colleagues, they made a patriotic martyr of Tojo, and ensured that the Tokyo War Crimes Trials would fail in their primary purpose, to bring home to the Japanese what their troops had done during the war. Bix's book presents, 55 years on, a robust case for the prosecution at the trial Hirohito never had to face while he was alive.

Describing the emperor's bizarre education - the best part of his book - Bix makes clear why Hirohito's war was lost from the start. Like all Japanese of his time, and now, he was brought up in the spirit of the Chinese sage Confucius, who taught benevolence, respect for parents and authority, and the idea that a ruler should govern by setting a good example. But on top of Confucius, Hirohito, as the future head of the Japanese armed forces, was taught the blood-thirsty samurai warrior code, which is anything but kindly; and, in addition, the racist Shinto superstition that all Japanese, and no one else, descend from the Sun Goddess, whose hereditary high priest was Hirohito himself. This mixture was not only contradictory and unstable, but guaranteed that Japan's attempts to build an empire would rouse a stubborn resistance in other Asians that no amount of samurai swordplay could over-come. It also made Hirohito the front-god for the Japanese military, shielding it from criticism at home, a role he conspicuously played, even if he did not personally invent the system that justified it.

Thus far, Bix is on solid ground. Hirohito certainly collaborated with the hard-bitten generals of the Japanese High Command, although Bix and other researchers have found few cases in which he gave a direct order. The basis of Japan's empire was laid in the 1895 war with China six years before Hirohito was born, the war with Russia, when he was four years of age, in which Japan (with enthusiastic British backing) won the right to station troops in the Chinese province of Manchuria, and the annexation of Korea, when he was nine.

The schoolboy Hirohito stepped into a role created for him by others whose agenda - an empire in Asia - he went along with; most emperors are happy to see the bounds of empire set wider and wider. Many of the incidents in Manchuria and China south of the Great Wall which spread the fighting were, Bix concedes, staged by undisciplined majors and colonels - encouraged, he argues, by Hirohito's failure to punish earlier insubordinations.

Bix is weaker on what happened after Japan's surrender, which was, he agrees, decided by Hirohito. There is a much simpler explanation than Bix's strained attempt to show that Hirohito was secretly trying to revive Japanese militarism. It is that Hirohito was a born collaborator - loyally collaborating with Japan's expansion-minded generals and admirals, just as he collaborated, after the war, with the autocratic American general who followed them. Brought up to be a front for militarists, Hirohito had his own heir, Akihito, educated as a pacifist, and approved when his son wanted to marry the Christian-educated Michiko Shoda, now Japan's much-admired empress.

A case can be made that Hirohito did his cautious best in the second half of his long reign to undo some of the evil to which he put his name in the first half - and the Queen did receive him. The Duke of Edinburgh went to his funeral in February 1989, but no one present, myself included, could decide whether Prince Philip bowed or not, the ever cautious British compromise.

But it is not Americans, Australians or the British who need to reach a verdict on Hirohito's legacy. It is for the Japanese themselves to decide his place in their violent, tragic past. This book is the case the 124th Emperor of Japan will one day have to answer before the court of history.