"Down this road on a summer day in 1944 the soldiers came. Nobody lives there now. They stayed only a few hours. When they had gone, a community which had lived for a thousand years was dead." These are the opening words of perhaps the most successful historical documentary series ever made. Read by Laurence Olivier, scripted by Neal Ascherson, master-minded by Jeremy Isaacs, their evocation of the village of Oradour-sur-Glane  obliterated by the Waffen-SS in June 1944  stood for "thousands and thousands of other martyrdoms" across Europe and beyond. Just before the recording, Ascherson cut the adjective "German" before "soldiers"  giving that opening sentence of death a fittingly universal resonance.

The World at War was screened on ITV on 26 Wednesday evenings between October 1973 and May 1974  the year of the oil crisis, the miners' strike and Ted Heath's three-day week. Almost 40 years on, it is still being watched around a very different world via DVDs and TV repeats.

Using the production archives held at the Imperial War Museum, supplemented by numerous interviews, Taylor Downing has constructed a fascinating story. The World at War was ITV's answer to The Great War, which had launched BBC2 in 1964. More precisely it was Isaacs's answer because, as this book shows, the series was his creature from start to finish. The BBC had been trying to lure back Isaacs (a former editor of Panorama) to do this obvious sequel to The Great War, but their pussy-footing gave him both the reason and the opportunity to sell the idea to his bosses at Thames Television. They agreed a budget of £440,000 (about £5m today, and half of what was eventually spent).

Isaacs always intended that The World at War, true to its title, should break out of the "finest hour" framework that still held sway in Britain. But Noble Frankland, the energetic director of the Imperial War Museum and series historical adviser, pushed Isaacs further, urging more coverage of the war against Japan and insisting that it was the Red Army, not the British and Americans, that really wore down Hitler's Wehrmacht.

One feature of the series was the use of extended interviews with ordinary people  a technique Isaacs imported from current affairs journalism. The interviewees were ferreted out by skilled researchers such as Sue McConachy, who cultivated Hitler's secretary Traudl Junge for a whole year to get her to speak for the first time on camera about the Führer's final days.

Meanwhile other researchers unearthed some 3.5m feet of archive film  all catalogued, in a pre-computer age, on 6in x 4in index cards. Gambling on a big hit, Thames acquired world rights in perpetuity for all the film it screened, a huge expense up front but ensuring the profits that still accrue to this day. Isaacs signed up the composer Carl Davis for the music, and pressure built up to find an "A-list actor" to be the voice of The World at War  someone even bigger than Michael Redgrave, who had narrated The Great War. That could mean only Olivier.

Having just played James Tyrone, the gruelling lead in Long Day's Journey into Night, Olivier was often tired and testy at recording sessions. Narrating a film, even for a fat fee, is essentially a supporting role, and Olivier was full of his own historical ideas and often over the top in delivery, with annoying verbal idiosyncracies such as his pronunciation of "Staleen". Yet Olivier's name and voice undoubtedly helped to sell the series  some American newspapers billed it as his take on the war.

Olivier was at his best in "Red Star", a moving depiction of the Soviet people's resistance to the Nazis, in which haunting archive footage and interviews with survivors were combined with Russian poetry and music. This came as a revelation to many British viewers brought up on Churchill's memoirs of the war, from which the Red Army was frozen out.

Even more important was "Holocaust", an episode that Isaacs (the son of Glaswegian Jews) had wanted to direct, before deciding he was too emotionally involved. Instead, Michael Darlow constructed a deliberately low-key film around a small number of interviews with people whom Downing calls "the most extraordinary victims and the most ordinary perpetrators", among them Himmler's adjutant. This was the first time a "detailed, analytical, no-holds-barred" programme about the Nazi genocide had been shown to British viewers.

As Downing acknowledges, The World at War was hardly definitive. A few months after it finished, the wraps came off the code-breakers of Bletchley Park  making several of the films somewhat passé, especially "Wolfpack" about the battle of the Atlantic. China's experience of the war, in which perhaps 15 million people died, is conspicuous by its absence; likewise Yugoslavia, an omission that seemed more glaring in the 1990s. But all historical analysis, in books or films, is partly a product of its time and place. The World at War was and remains a landmark, the significance of which we can better appreciate through this clear, incisive and readable book.

 David Reynolds's In Command of History: Churchill Fighting and Writing the Second World War is published by Penguin.