One result of the fall of communism and the break-up of the Soviet Union was the emergence of hitherto unknown protagonists and witnesses of Nazi atrocities. Haunted by the idea that what happened once could happen again,Laurence Rees went east to interview onlookers, surviving Jews, beneficiaries of appropriated wealth and property, and mass murderers themselves, only now telling what they did in 1940-45.

The book of the BBC television series sets these survivors in the context of the tide of unreason that was Nazi Germany, in an attempt to understand how so many ordinary people could have been implicated in the killing of decent civilians in hundreds and thousands, and in cold blood.

For we cannot take refuge in the belief that it was the derangement of one man which caused the Holocaust - atrocities were committed by ordinary people.

Hitler presided over chaos and entropy; he knew what outcomes he wanted but had no interest in how they were to be achieved. There was no hierarchy of power and no chain of control. The only policy was “working towards the Fuehrer”, which left bickering sycophants guessing what Hitler wanted, and achieving it by any means to hand.

Some of Hitler’s henchmen, like Schacht, his 1930s economic minister who beat inflation and got Germany back to work, succeeded. Others, like Ribbentrop in foreign policy, incompetently and spectacularly failed, delivering in 1939 to a Fuehrer who admired Britain and feared Bolshevism, the wrong ally and the wrong war.

Eventually entropy triumphed, as events in Eastern Europe dreadfully testify. Rees demonstrates how, when invading Poland and acquiescing in the Russian takeover of the Baltic states, the Nazis created chaos with their attempted “Germanization” of the areas they annexed.

Massive movements of population were launched without forward planning or consideration of the consequences. Ethnic Germans from the Baltic displaced Poles. Poles, and more particularly Jews, were regarded as a source of wealth to be taken by right. Destitution produced discord and disaster as re-settlement on a huge scale lurched out of control and all vestige of respect for law and human dignity vanished.

The perpetrators stare out from these pages, a level stare, shockingly normal. Their testimony is matter-of-fact.

“When we had finished the shooting,” says one Lithuanian, Petras Zelionka,speaking of a mass slaughter of Lithuanian Jews, “we had lunch at a restaurant in Krakes.”

Some witnesses describe onlookers holding up their children to watch people being killed in the streets; a few speak of their shame in failing to intervene. Most chilling is the testimony of those who even now subscribe to the self-serving racism and bigotry with which they justified their actions.

Some old Germans even now believe themselves a superior nation shamefully traduced by a conspiracy of Jews. Zelionka, too, says: “They were only the Jews, no one was our countryman. They were all Jews.”

These bleak accounts are punctuated by stark black and white photographs,as the camera captures violence in city streets and horror in the Eastern forests. The silent, terrified, pleading scream of a violated Jewish woman,Polish men hanging dead from a beam, women running with children in their arms, troops shooting civilians in the back, all are frozen into harsh white sunlit relief, the unspeakable etched forever in everyday places.

None of this, says Rees, must be forgotten or obscured. Knowledge will help prevent repetition. “Ten years ago I couldn’t have spoken like this,” agrees one eminent German interviewee, “but now, well . . . I will be dead soon. It is time to tell all of the truth.”