By Peter Hennessy and Sanchia Berg
Whitehall historian Professor Peter Hennessy explains what the government War Book contains and how it came to be published
What was once the most secret of all British government documents has been released in full.
During the Cold War, civil servants used to rehearse the end of the world - what would happen if deterrence failed and nuclear war became inevitable.
David Young, a former senior civil servant in the MoD, recalls one of the "transition to war" exercises of the early 1970s.
"R hour would be the final release of nuclear weapons. There may have been an earlier tactical use of nuclear weapons but R hour was everything that's left goes".
"I participated in one R hour in the early hours of the morning and I remember reporting on it afterwards through the newly installed closed circuit TV. And foolishly saying because of the day of the week - 'There we are, R hour, sic transit gloria Thursday'.
The exercise was taken very seriously - and jokes were frowned on, even if they were elegant puns on Latin phrases - "sic transit gloria mundi" is, of course, "so passes the world's glory".
Mr Young said: "I subsequently learned the Foreign Secretary Alec Douglas Home said, 'Who is that very foolish young man?'"
The transition to war exercises took place every two years during the Cold War, and were an integral part of deterrence.
If the Soviet Union knew the West had very detailed plans to deal with nuclear war, the argument went, it would be less likely to attack. On that occasion, in the 70s, Mr Young played the role of defence secretary - as ministers themselves were not encouraged to take part.
David Young and Sir David Omand recall taking part in the exercises
"They would be disinclined to play by the rules. Some of them quite liked talking, so you'd get behind time, and there would be a fear that if they showed a reluctance to do what the military believed was necessary that this would weaken deterrence," he said.
The purpose was to test the government War Book. To see, for instance, if the plan gave the BBC enough notice to staff its wartime bunker, or if the plans to round up "subversives" were in the right order.
Parts of the War Book have been released before but this is the first time the whole document has been declassified.
Having the full War Book means it can be decoded to reveal what would happen and when. To get the nation from a peacetime to a wartime footing and beyond was a remarkable enterprise.
It must have been one of the most terrible jobs in Whitehall - literally requiring people in immense secrecy to peer into the abyss.
The scenarios for some of the exercises have already been released.
For instance, the one which began in late September 1968 includes the Soviet Union landing astronauts on the Moon.
Soviet 'Moon landing'
The file has mock daily briefings from the Joint Intelligence Committee on international events - the Home Front is covered by bulletins from civil defence officials in the Home Office. Every day the "cabinet" of civil servants would meet and decide which parts of the War Book to implement.
By 17 October, the day of the Soviet Moon landing, tension had risen. Czechoslovakian and Hungarian troops were said to be massing on the border with Austria. Soviet fighters had been harassing civil aircraft in the Berlin corridors, causing an American airliner to crash.
Meanwhile, in the UK, people were getting nervous. The mock news bulletins report that "letters are beginning to appear in newspapers asking when advice is going to be given to householders about protecting their homes".
And in the Cabinet Office, the mock cabinet implemented a number of War Book measures, including alerting civil servants who would have to man the regional bunkers. If the bombs dropped, these would have been the regional centres of government.
Warsaw Pact 'invasion'
That exercise, like the others, proceeded with grim inevitability. Warsaw Pact troops 'invaded' Austria, then West Germany, Finland, Turkey, Greece and Italy. There were reports that biological and chemical weapons had been used. Scandinavia was threatened.
Meanwhile, people started staying away from work and there was heavy demand at builder's yards for the materials to build household refuges, as well as long queues at food shops.
When the Soviet forces "attacked" Danish islands, people started moving out of British cities. There were anti-war demonstrations in Downing Street but they petered out as the Soviet advance continued
The mock cabinet implemented the last phases of the War Book - removing major art treasures from London to Edinburgh, discharging all but the acutely ill from hospital and setting up the machinery of justice in wartime.
The plans go into extraordinary detail - there is even one note to charge premiums for government reinsurance against war risk.
My favourite measure, the one which always aroused a lot of debate when we played these exercises, was the introduction of censorship for private correspondence
Sir David Omand
That exercise ends with R hour - the release of all nuclear weapons. David Young says his hairs still stand up when he thinks of it.
"That's not an easy decision to participate in. Even though you know it's just an exercise, but it makes you think."
Sir David Omand, latterly chairman of Whitehall's Security and Intelligence Committee, also took part in these exercises for many years, right up to the early 1990s. He once played the prime minister, an experience he described as "scary".
"My favourite measure, the one which always aroused a lot of debate when we played these exercises, was the introduction of censorship for private correspondence. You can imagine that was something that ministers would only agree to right at the very end when it was clear that war was inevitable."
Civil servants do not rehearse nuclear Armageddon any more. They still run planning exercises, though - usually involving a major terrorist attack.
'What do we do?'
Sir David was permanent secretary at the Home Office in 2000 when the fuel protests took place. He said when that started nobody had any real plans for dealing with it - as a result of which the first few days were chaotic.
"I recall Jack Straw as home secretary turning to me and saying, "This isn't working - what do we do?"
So they adapted plans for other emergencies.
"We took over the bunker, and installed a chief constable, and representatives of the oil companies, and some civil servants, and we built from scratch a crisis management machine.
And that's exactly what you don't want to have to do in a crisis because a lot of time's spent just organising who's going to talk to who and how it's going to work."