Relations between Russia and Germany have not been good since Vladimir Putin's nationalist sabre-rattling this summer, but they are about to get a whole lot worse.

A new film about to be released in Germany will force both countries to re-examine part of their recent history that each would much prefer to forget. Yet it is right that the ghastly truth should finally be acknowledged.

The movie, A Woman In Berlin, is based on the diary of the German journalist Marta Hillers and depicts the horror of the Red Army's capture of the capital of the Third Reich in April and May 1945.

Marta was one of two million German women who were raped by soldiers of the Red Army  -  in her case, as in so many others, several times over.

It was a feature of Russia's 'liberation' and occupation of eastern Germany at the end of World War II that is familiar enough to historians, but  which neither country cares to acknowledge took place on anything like the scale it did.

For Russia, the episode besmirches the fine name of the Red Army that had fought so hard and suffered so much in its four-year campaign against the Wehrmacht.

The courage and resilience of the ordinary Russian in what they called the Great Patriotic War is incontestable, and for every five German soldiers killed in action in the whole of World War II, four died on the Eastern Front.

Yet the knowledge that the victorious Red Army committed mass rape across Prussia and eastern Germany as they closed in on Berlin degrades its reputation, which is unacceptable to many Russians today.

When the historian Antony Beevor wrote about it in his book Berlin: The Downfall, the Russian ambassador to London, Grigory Karasin, accused him of 'an act of blasphemy', saying: 'It is a slander against the people who saved the world from Nazism.'

Similarly, living Germans do not want the events that humiliated and violated them, their mothers and grandmothers to be held up to public examination, as this movie promises to do.

For many German women, the memory was something they sublimated and never spoke about to their husbands returning from the front.

It was the great unmentionable fact of 1945, which is coming out not just in history books, but in front of a mass, international audience. Painful memories of gross sexual abuse are being dragged out and held up to the pitiless witness of the silver screen.

Furthermore for the Germans, there is also the added sense that had it not been for Operation Barbarossa, Hitler's invasion of the USSR, these crimes would never have been committed against German womanhood in the first place.

Three million German troops crossed the Soviet border in June 1941 in an attempt to extirpate the Russian state, and the Nazi commitment to

Total War produced atrocities so terrible that they were bound to be avenged once the Red Army reached German soil.

As so often in war, it was to be defenceless women, girls and even elderly ladies who were to pay in pain and outrage for the crimes of their male compatriots.

Many had abortions or were treated for the syphilis they caught. And as for the so-called Russenbabies  -  the children born out of rape  -  many were abandoned.

In his fine new book, World War Two: Behind Closed Doors, the historian Laurence Rees points out that although rape was officially a crime in the Red Army, in fact, Stalin explicitly condoned it as a method of rewarding the soldiers and terrorising German civilians.

Stalin said people should ' understand it if a soldier who has crossed thousands of kilometres through blood and fire and death has fun with a woman or takes some trifle'. 

On another occasion, when told that Red Army soldiers sexually maltreated German refugees, he said: 'We lecture our soldiers too much; let them have their initiative.'

While Stalin condoned rape as an instrument of state military policy, his police chief Lavrenti Beria was a serial rapist.

An American diplomat, Beria's bodyguard and the Russian actress Tatiana Okunevskaya all bore witness of his methods of grabbing women off the street and shoving them into his limousine and then his bed.

'You are a long way from anywhere, so whether you scream or not does not matter,' Beria would tell the women when he got them back to his dacha. 'You are in my power now. So think about that and behave accordingly.'

More than 100 school-aged girls and young women were drugged and raped by the man who ran the NKVD, the feared forerunner to the KGB.

'All of which means, of course, that if reports of Red Army soldiers raping women in eastern Europe were sent to the NKVD in Moscow, they finally reached the desk of a rapist himself,' says Rees.

The rape of Berlin's female population  -  anyone between the ages of 13 and 70 was in danger  -  was cruelly vicious.

In one heart-breaking incident, a Berlin lawyer, who had somehow protected his Jewish wife from persecution throughout the Nazi period, was shot trying to protect her from rape by Red Army soldiers. As he lay dying of his wounds, he saw his wife being gang-raped.

'They do not speak a word of Russian, but that makes it easier,' one Red Army soldier wrote in a letter home in February 1945. 'You don't have to persuade them. You just point a revolver and tell them to lie down. Then you do your stuff and go away.'

It was unusual for Red Army soldiers to admit to rape in letters home, which is why this new German film is likely to shock Russian patriotic sensibilities.

Nor did the Germany's surrender calm the Russians' behaviour, at least in the short term.

'Weeks before you entered this house, its tenants were living in constant fright and fear,' the rich German publisher Hans-Dietrich Muller Grote wrote to President Truman about the place he stayed in during the Potsdam conference of July 1945.

'By day and by night, plundering Russian soldiers went in and out, raping my sisters before their own parents and children, beating up my old parents. All furniture, wardrobes and trunks were smashed with bayonets and rifle butts, their contents soiled and destroyed in an indescribable manner.'

The Red Army's atrocities against women in Dresden in the spring of 1945, a city that had already suffered heavily from Allied bombing, were carried out in a sickeningly systematic manner.

'In the house next to ours, Soviet troops went in and pulled the women on to the street, had their mattresses pulled out and raped the women,' recalled one inhabitant, John Noble.

'The men had to watch, and then the men were shot. Right at the end of the street, a woman was tied to a wagon wheel and terribly misused.

'Of course, you had the feeling that you wanted to stop it, but there was no possibility to do that.' Women going to and from work past Red Army pickets were routinely raped.

The historian Chris Bellamy believes that although there are no surviving written records to prove it, 'the hideous spectre of multiple rape was not only condoned, but, we can be pretty sure, legally sanctioned by the political officers speaking for the Soviet government'.

Nor is it true that rape was mainly carried out by reserve units following behind the front-line troops.

The Russian war correspondent Vassily Grossman was embedded with the elite front-line Eighth Guards Army which committed rape, as did at least one of his own war correspondent colleagues.

As well as the estimated two million rapes in Germany, there were between 70,000 and 100,000 in Vienna and anywhere from 50,000 to 200,000 in Hungary, as well as thousands in Romania and Bulgaria, which had been pro-Nazi, but also in Poland, Czechoslovakia and Yugoslavia, which had not been.

Indeed, as Beevor points out, the Red Army even raped <cite>Russian </cite>women who had been liberated from concentration camps, emaciated and wearing their prison uniform.

Overall, however, Russian soldiers tended to prefer plumper and better-fed women, and one diarist recorded the satisfaction felt by some Berlin women that these tended to be the wives of Nazi Party functionaries.

Vodka played a role, of course, but the worst of the behaviour was fuelled by sheer hatred and aggression, as well as a cold-hearted sense of deterrence for the future.

'It's absolutely clear that if we don't really scare them now, there will be no way of avoiding another war in future,' one Red Army soldier wrote at the time.

In a recently published book by the Professor of Modern History at Cambridge, Richard Evans, a young Russian officer is quoted recalling how when his unit overtook a column of fleeing German refugees: 'Women, mothers and their children lie to the right and left along the route, and in front of each of them stands a raucous armada of men with their trousers down.

The women, who are bleeding or losing consciousness, get shoved to one side, and our men shoot the ones who try to save their children.'

A group of 'grinning' officers ensured that 'every soldier without exception would take part'.

Evans records: 'Rape was often accompanied by torture and mutilation and frequently ends in the victim being shot or bludgeoned to death. The raging violence was undiscriminating.'

The insistence on the men watching the rapes was deliberate policy, intended 'to underline the humiliation'.

Underlying all this foul inhumanity was the way the German Army and its allies had behaved during its invasion of Russia.

That was clearly not the only explanation  -  it doesn't explain why the Red Army raped Poles, Czechs and Yugoslavs and even Russian women, for example  -  for that one has to delve deep into the darkest recesses of the male psyche.

Yet the four years of life-and-death conflict did brutalise the Soviet peasant-soldier and taught him to behave like a beast against the women of his enemies once he was given the chance. The Soviet-German war lasted 1,418 days without respite, and for sheer horror there is nothing to equal it in the long and monstrous annals of human warfare.

'It eliminated pity, abandoned any constraint, mocked even a semblance of legality,' wrote its foremost historian Professor John Erickson. The Germans drew up plans to exterminate or deport no fewer than 50 million Slav untermensch (subhumans) and as soon as they had attacked they instituted procedures for achieving this. In the end, no fewer than 27 million citizens lost their lives.

The Geneva Conventions were ripped up, as it was stated that no German soldier would be prosecuted for any 'ideologically motivated' murder of civilians.

Hitler's Vernichtungskampf (war of annihilation) against the Slavs merged into his Rassenkampf (war of racial extermination) against the Jews and Communists to create a Continent-wide slaughter.

Behind the advancing Wehrmacht, which won victory after victory in the first six months, were a series of Einsatzgruppen (action squads), whose 'special task' it was to liquidate Jews, Communists, partisans, PoWs, the disabled and anyone else thought to be 'enemies of the Reich'.

In forests across eastern and southern Russia, Ukraine, Poland and the Baltic states, the populations of villages and towns were escorted to places of execution, ordered to dig their own shallow graves and then shot.

An estimated 1.5million innocent human beings are believed to have perished in this way. Yet, as Erickson puts it: 'Machine-guns and rudimentary gas vans could not cope with the demands imposed by mass extermination, prompting the search for a capacious gas chamber the like of Auschwitz.'

The 'scorched earth' policy adopted by the Wehrmacht led to millions more dying, and during the 900-day siege of Leningrad human flesh was semi-openly sold in the street. Small wonder that when 'Ivan' had his chance for revenge, he was going to take it in as gross and bestial a manner as possible.

The fact that the women of Germany were largely innocents, swept up in the horror rather than being responsible for it, meant nothing to an army that had lost 13.5 million casualties at the Germans' hands.

And what of the Allied advance in the West across Germany? It was not unknown for cases of rape to be reported, but they were considered a serious offence and punished accordingly.

The fact that today Germany is a peaceful democracy, indeed, in many ways a model country, can in large part be put down to the Red Army's monstrous invasion, but also to the ruthless bombing of its cities and towns by the British and U.S. air forces from 1941 to 1945.

The aggressive soul of Germany that had launched no fewer than five wars in the 75 years after 1864 was cut out by the experiences of World War II.

If made sensitively, this new film might be able to reconcile the two countries as they come to terms with the crimes committed in the first half of the Forties.

Equally, the capacity for reciprocated resentment is ever present. A Woman In Berlin is thus cinematic gunpowder. Yet neither side should hide from the harsh historical truths it tells, however unpalatable they may seem.

•  Andrew Robert's latest book is Masters And Commanders: How Roosevelt, Churchill, Marshall And Alanbrooke Won The War In The West (Allen Lane, £25).