In his new book, he reveals for the first time that he copied an additional 8000 pages of top-secret material from his work for the White House and the RAND Corporation, a military research and development business that advises the US armed forces.
Much of his work at the RAND Corporation involved drawing up a secret plan, under president Dwight Eisenhower, for a nuclear war.
Mr Ellsberg asserts that much of what he learnt of the dire threats US nuclear strategy posed in the late ’50s and early ’60s is still true today.
“The basic elements of American readiness for nuclear war remain today what they were almost 60 years ago: Thousands of nuclear weapons remain on hair-trigger alert, aimed mainly at Russian military targets including command and control, many in or near cities,” he writes in the book.
US policy has long stated that the primary purpose of its nuclear weapons program is to deter a first strike against the American mainland, but Mr Ellsberg says this is a “deliberate deception”.
“The nature, scale and posture of our strategic nuclear forces has always been shaped by the requirements of quite different purposes: to attempt to limit the damage to the United States from Soviet or Russian retaliation to a US first strike against the USSR or Russia,” he writes.
In other words, the enormity of the US nuclear arsenal is designed to be effective in initiating nuclear conflict and deterring any retaliation.
Mr Ellsberg writes that US nuclear plans have always been drawn up with the aim of striking first under all circumstances.
Successive presidents, Donald Trump included, have declined to change US military policy to “no first use”, which is a pledge not to use nuclear weapons unless first attacked by an enemy using its own nukes.
Mr Ellsberg argues that this creates an implied threat of nuclear attack on every state that might come into conflict with the US, such as North Korea, and only serves to encourage nuclear weapons proliferation.
“Indeed, it has encouraged proliferation in states hoping either to counter these American threats or to imitate them,” he writes.
“Of course, our insistence on maintaining an arsenal of thousands of weapons, many on alert, a quarter century into the post-Cold War era, nullifies our advice to most other states in the world that they ‘have no need’ or justification for producing a single nuclear weapon.”
One of the more alarming parts of the book is the revelation of what Mr Ellsberg calls “one of our highest national secrets” – that a number of people in the US have the delegated authority to pull the trigger on nuclear weapons.
“With respect to deliberate, authorised US strategic attacks, the system has always been designed to be triggered by a far wider range of events than the public has ever imagined,” Mr Ellsberg writes.
“Moreover, the hand authorised to pull the trigger on US nuclear forces has never been exclusively that of the president, nor even his highest military officials.”
Mr Ellsberg discovered in the 1950s that Mr Eisenhower had secretly delegated authority to order nuclear attacks to military commanders under certain circumstances. The policy was continued by subsequent presidents and Mr Ellsberg says it has been carried on “almost certainly” in the Trump administration.
Russia has a similar protocol, known as the Dead Hand system, which would ensure delegated authority to retaliate to a US strike in the event that the country’s commanders were taken out.
“An urgent reason for enlightening the world’s public on this reality of the nuclear era is that it is virtually certain that this same secret delegation exists in every nuclear state, including the new ones: Israel, India, Pakistan and North Korea.
“How many fingers are on Pakistani nuclear buttons? Probably not even the president of Pakistan knows reliably.”
Mr Ellsberg added that reports on US plans to “decapitate” the North Korean leadership probably encouraged North Korea to establish its own Dead Hand system, to assure retaliation to such an attack.
He also writes that the US nuclear system is “prone to false alarms, accidents and unauthorised launches” that could have catastrophic consequences.
“What none of us knew at the time … were the phenomena of nuclear winter and nuclear famine, which meant that a large nuclear war of the kind we prepared for then or later would kill nearly every human being on Earth.”
The fact that the world’s nuclear powers essentially have the ability to wipe humankind off the planet is what Mr Ellsberg calls the “Doomsday Machine”.
Borrowing the phrase from Stanley Kubrick’s 1964 satire Dr Strangelove Or: How I Learned To Stop Worrying And Love The Bomb, he uses the concept for a more serious purpose.
“An American Doomsday Machine existed in 1961 – and had for years – in the form of pre-targeted bombers on alert in the Strategic Air Command, soon to be joined by Polaris submarine-launched missiles. Although this machine wasn’t likely to kill outright or starve to death literally every last human, its effects, once triggered, would come close enough to that to deserve the name Doomsday.”
Mr Ellsberg’s book arrives at time when nuclear war with North Korea is closer than ever before, according to former US Joint Chiefs of Staff chairman Mike Mullen.
Mr Trump has reiterated his view that “nothing is off the table” in terms of responding to the threat from North Korea and other enemies.
Mr Ellsberg expresses increased anxiety about Mr Trump having the nuclear codes due to his erratic personality.
“Other major candidates and presidents taking the same position have aroused less unease than Donald Trump who, along with being unusually volatile and thin-skinned, has explicitly embraced a deliberate penchant for unpredictability, evident not only during the campaign but also while president.”
Mr Ellsberg sums up by saying the world’s nuclear build-up is a “chronicle of human madness”.
“Most aspects of the US nuclear planning system and force readiness that became known to me half a century ago still exist today, as prone to catastrophe as ever but on a scale, as now known to environmental scientists, looming vastly larger than was understood then,” he writes.
“The present risks of the current nuclear era go far beyond the dangers of proliferation and non-state terrorism that have been the almost exclusive focus of public concern for the past generation and the past decade in particular.
“The hidden reality I aim to expose is that for over 50 years, all-out thermonuclear war – an irreversible, unprecedented and almost unimaginable calamity for civilisation and most life on Earth – has been … a catastrophe waiting to happen. And that is still true today.”
“The chance that this system could explode ‘by mistake’ or unauthorised action during a crisis – as well as by the deliberate execution of nuclear threats – taking much of the world with it, has always been an unconscionable risk imposed by the superpowers upon the population of the world,” he writes.
Mr Ellsberg learnt in the ’60s that a full-scale nuclear war could wipe out as much as a third of the Earth’s population.
We know now that the eventual consequences could be even worse.
Experts argue that smoke from the fires caused by nuclear bombings on cities would engulf the globe, blocking out sunlight, freezing farmlands and eventually causing worldwide famine.