In 1945, as at the end of all wars, the victor powers spun the conflict's history to serve the interests of their elites. Wartime propaganda thus achieved an extraordinary afterlife. As Vladimir Putin showed yesterday, the Great Patriotic War remains a key political resource in Russia. In Britain and the US, too, a certain idea of the second world war is enthusiastically kept alive and less flattering memories suppressed.

Five years ago, Robert Lilly, a distinguished American sociologist, prepared a book based on military archives. Taken by Force is a study of the rapes committed by American soldiers in Europe between 1942 and 1945. He submitted his manuscript in 2001. But after September 11, its US publisher suppressed it, and it first appeared in 2003 in a French translation.

We know from Anthony Beevor about the sexual violence unleashed by the Red Army, but we prefer not to know about mass rape committed by American and British troops. Lilly suggests a minimum of 10,000 American rapes. Contemporaries described a much wider scale of unpunished sex crime. Time Magazine reported in September 1945: "Our own army and the British army along with ours have done their share of looting and raping ... we too are considered an army of rapists."

The British and American publics share a sunny view of the second world war. The evil of Auschwitz and Dachau, turned inside out, clothes the conflict in a shiny virtue. Movies, popular histories and political speeches frame the war as a symbol of Anglo-American courage, with the Red Army's central role forgotten. This was, we believe, "a war for democracy". Americans believe that they fought the war to rescue the world. For apologists of the British Empire, such as Niall Ferguson, the war was an ethical bath where the sins of centuries of conquest, slavery and exploitation were expiated. We are marked forever as "the good guys"and can all happily chant "Two world wars and one world cup."

All this seems innocent fun, but patriotic myths have sharp edges. The "good war" against Hitler has underwritten 60 years of warmaking. It has become an ethical blank cheque for British and US power. We claim the right to bomb, to maim, to imprison without trial on the basis of direct and implicit appeals to the war against fascism.

When we fall out with such tyrant friends as Noriega, Milosevic or Saddam we rebrand them as "Hitler". In the "good war" against them, all bad things become forgettable "collateral damage". The devastation of civilian targets in Serbia or Iraq, torture at Abu Ghraib and Guantánamo, the war crime of collective punishment in Falluja, fade to oblivion as the "price of democracy".

Our democratic imperialism prefers to forget that fascism had important Anglo-American roots. Hitler's dream was inspired, in part, by the British Empire. In eastern Europe, the Nazis hoped to make their America and Australia, where ethnic cleansing and slave labour created a frontier for settlement. In western Europe, they sought their India from which revenues, labour and soldiers might be extracted.

American imperialism in Latin America gave explicit precedents for Germany's and Japan's claims of supremacy in their neighbouring regions. The British and Americans were key theorists of eugenics and had made racial segregation respectable. The concentration camp was a British invention, and in Iraq and Afghanistan the British were the first to use air power to repress partisan resistance. The Luftwaffe - in its assault on Guernica, and later London and Coventry - paid homage to Bomber Harris's terror bombing of the Kurds in the 1920s.

We forget, too, that British and US elites gave aid to the fascists. President Bush's grandfather, prosecuted for "trading with the enemy" in 1942, was one of many powerful Anglo-Americans who liked Mussolini and Hitler and did what they could to help. Appeasement as a state policy was only the tip of an iceberg of practical aid to these dictatorships. Capital and technology flowed freely, and fascist despots received dignified treatment in Washington and London. Henry Ford made Hitler birthday gifts of 50,000 marks.

We least like to remember that our side also committed war crimes in the 1940s. The destruction of Dresden, a city filled with women, children, the elderly and the wounded, and with no military significance, is only the best known of the atrocities committed by our bombers against civilian populations. We know about the notorious Japanese abuse of prisoners of war, but do not remember the torture and murder of captured Japanese. Edgar Jones, an "embedded" Pacific war correspondent, wrote in 1946: "'We shot prisoners in cold blood, wiped out hospitals, strafed lifeboats, killed or mistreated enemy civilians, finished off the enemy wounded, tossed the dying into a hole with the dead, and in the Pacific boiled flesh off enemy skulls to make table ornaments."

After 1945, we borrowed many fascist methods. Nuremberg only punished a handful of the guilty; most walked free with our help. In 1946, Project Paperclip secretly brought more than 1,000 Nazi scientists to the US. Among their ranks were Kurt Blome, who had tested nerve gas at Auschwitz, and Konrad Schaeffer, who forced salt into victims at Dachau. Other experiments at mind control via drugs and surgery were folded into the CIA's Project Bluebird. Japan's Dr Shiro Ishii, who had experimented with prisoners in Manchuria, came to Maryland to advise on bio-weapons. Within a decade of British troops liberating Belsen, they were running their own concentration camps in Kenya to crush the Mau Mau. The Gestapo's torture techniques were borrowed by the French in Algeria, and then disseminated by the Americans to Latin American dictatorships in the 60s and 70s. We see their extension today in the American camps in Cuba and Diego Garcia.

War has a brutalising momentum. This is the moral of Taken By Force, which shows how American soldiers became increasingly indiscriminate in their sexual violence and military authorities increasingly lax in its prosecution. Even as we remember the evils of nazism, and the courage of those who defeated it, we should begin to remember the second world war with less self- satisfaction. We might, in particular, learn to distrust those who use it to justify contemporary warmongering.

· Richard Drayton is senior lecturer in history at Cambridge University.