In 1964, with more than ten years as a nuclear planner already behind him, Daniel Ellsberg went with a colleague to see a movie. They staggered out an hour and a half later, pale and shaking, convinced that, instead of the promised satire, they had just viewed a documentary.

At the heart of Stanley Kubrick’s Dr Strangelove is “the Doomsday Machine”, a device set to trigger automatically should the Soviet Union be attacked by even a single bomb, thus guaranteeing that the balance-book of human achievement and civilisation be set back to zero.

It was decades more before Ellsberg recognised just how literally such a system was in place – known in the West as Perimeter, and to the Soviets by the Russian phrase that means “Dead Hand” – and just how close to wilful omnicide the world sat.

That particular revelation still lay in wait for the world’s most thoughtful and self-aware whistleblower. What worried Ellsberg and his colleague in 1964 was the degree to which Kubrick had penetrated their ongoing nightmare, which was the possibility of a “rogue” commander launching a nuclear attack without full presidential or other governmental authority. Again, little had they known.

There is a moment during every presidential inauguration since the 1950s, sometimes caught on camera, where, mid-oath, the Secret Service man charged with carrying the infamous nuclear “football” switches his gaze from the face of the outgoing president to that of his successor. It’s a piece of

courtly-comic theatre, but then, so is the “football” itself. As a young RAND consultant, studying American military preparedness, Ellsberg was shocked to discover the extent to which nuclear launch decisions had been pre-delegated to area commanders. It isn’t made clear whether the major factor involved was Ike’s heart or his golf diary. Either way, it now seems clear that, from the Eisenhower administration onwards, ultimate control over a vast arsenal of thermonuclear weapons (each of which contains the equivalent of the Nagasaki bomb as a percussion cap) is devolved to theatre command.

Needless to say, reading The Doomsday Machine today feels a little different from reading it in 2015 might have been. Donald Trump is right out of the Merkin Muffley school of presidency but it may well be that our future as a species has always lain in the hands of General Buck Turgidsons and Brigadier General Jack D Rippers.

The all-too-real-life Curtis LeMay, who headed Strategic Air Command for a decade (and then stood as George Wallace’s running-mate in 1968, just in case anyone had failed to recognise how crazy he was), was the inventor of the disputed “Stone Age” quote about modern strategic bombing; whether he used it with the word “could” or the word “should” is the matter at issue, but he never seemed remotely hesitant about pushing the button.

Ellsberg makes an important point about the advent of nuclear weapons. Even before Hiroshima, the Americans – learning much from their British allies – knew how to kill thousands of citizens by urban firebombing.

The fission bomb just put that capacity into convenient form. The bomb was never primarily intended for Japan, though. It was always intended for use against the Soviet Union, and it is either a small miracle or a major strategic miscalculation that it wasn’t so used before the Russians got hold of their own “deterrent”.

Either way, the world has lived with it ever since, largely in ignorance of the realities.

Perhaps the most chilling thing in Ellsberg’s unflaggingly urgent book is a small diagram on page two, headed “TOP SECRET – SENSITIVE Joint Chiefs of Staff For the President’s Eyes Only”. It shows a single rising line, plotting nuclear fatalities in the event of all-out war with the Soviet Union, and no other kind was ever seriously contemplated.

The numbers start at 275 million, rising to 325 million in six months. But, just to be clear, this figure was arrived at without consideration of either fire or smoke, which as any fule kno are occasional by-products of socking great bombs.

By 1964, when Ellsberg and his colleague staggered out of that theatre into the daylight flash of revelation, the world had already come close to destruction. Everyone knows the story of the Cuban missile crisis. It makes for great Hollywood and Roger Donaldson’s 2000 movie Thirteen Days is perhaps the natural antidote to Dr Strangelove. Hunky old Bruce Greenwood as JFK saves the world, with a lot of help from Kevin Costner.

There’s bluff and counter-bluff, Wild West tension and a side order of moral agony. The conclusion is that world leaders aren’t monsters but thoughtful men who understand the stakes, and indeed, Ellsberg reproduces the words of both Kennedy and Nikita Khrushchev suggesting exactly that.

But what hardly anyone knows is that the Cuba crisis had a hidden aspect, a deadly stalking match between American warships, some “practice” depth charges and a Soviet submarine in which the temperature was rising to 140 degrees and the carbon dioxide to toxic levels: a Soviet submarine with a nuclear torpedo.

In the event, two of the officers pre-delegated to authorise launch gave the go-ahead, while the third, Vasili Arkhipov, who happened to be travelling with them, did not.

Had the torpedo been launched and American ships vaporised or irradiated, the Third World War would have begun and mankind, plus the majority of other species, ceased to exist. Military strategists at the time – and that, of course, includes the younger Ellsberg – put the risk of nuclear conflict at one in a hundred. We now know that we avoided catastrophe by a “handbreadth”.

This is, at some level, comic. I laughed a good many times when reading The Doomsday Machine, though rarely in a good way. The one out-and-out joke in the book is the proposal that a grid of powerful rockets, laid horizontally and fired simultaneously, might slow the Earth’s orbit for a moment or two and put the Russian missiles off target. That it would also destroy every structure on the planet and unleash giant tsunamis to wash away the rest doesn’t seem to have been considered. Hilarious.

Yet, it is also somewhat reassuring that throughout the nuclear age intelligent and principled men on both sides, however deluded by position, patriotism, principle, however blinded by overlapping layers of secrecy, continued to ask questions, run scenarios, leak information to the public.

As whistleblowers go, Ellsberg has always punched exactly at his weight.

He is an unabashed insider, complicit with much of what he now condemns, but unflinchingly willing to reveal

his mistakes and failures of courage.

The release of the “Pentagon Papers” in 1971, now the subject of one of 2018’s big films The Post, made Ellsberg an outlaw and a liberal folk-hero. The Doomsday Machine takes him to a whole new level. Climate change may or may not be a Chinese hoax, may or may not be man-made, reversible or irreversible. There is, however, still time and ample chance to defuse the bomb that has been ticking under us for 60 years.