Daniel Ellsberg, who has died aged 92, was the most important whistleblower of our times. His 1971 leaking of what became known as the Pentagon Papers showed conclusively that virtually everything the American public had been told by its leaders about the Vietnam war, from its origins to its current conduct, was false.The leak itself did not end the war, and Ellsberg regretted not having come forward years earlier. He spent the rest of his life as a peace activist, encouraging others on the inside to reveal government malfeasance, and supporting those who did, including the 2003 GCHQ whistleblower Katharine Gun. But his leaks did result in a landmark decision in favour of freedom of the press, and, ironically, led to the downfall of the US president Richard Nixon. It is not unreasonable to set Ellsberg’s leak alongside President John F Kennedy’s assassination as the ground zero of today’s distrust of politics.Before working on the Pentagon Papers, officially a study titled A History of Decision-Making in Vietnam 1945-68 commissioned from the Rand Corporation research organisation by the secretary of defense Robert McNamara, Ellsberg had spent two years at the US embassy in Saigon, advising on General Edward Lansdale’s “pacification” programme. As he sifted through the material gathered for the report, including evaluations which deemed the war unwinnable, he realised the enormity of the political fraud.He began copying the documents, with the help of a former Rand colleague Anthony Russo, and in 1971, as the US extended the war with bombings of Laos and Cambodia, resolved to make them public. The chair of the senate foreign relations committee, William Fulbright, turned him down, as did the Washington Post’s editor Ben Bradlee and owner Katharine Graham; Graham was close to the secretary of state Henry Kissinger, who had known Ellsberg at Harvard; he advised her Ellsberg was “unbalanced and emotionally unstable”. Matthew Rhys played Ellsberg in the 2017 film The Post which loosely covers those events.Neil Sheehan of the New York Times was a reporter Ellsberg admired in Vietnam; Sheehan convinced the Times to take the papers, the first instalment of which revealed that the Gulf of Tonkin incident, the casus belli which launched full-scale US participation in the conflict, had been bogus.The Nixon administration obtained an injunction prohibiting further publication; the supreme court’s overturning of that injunction, dismissing the idea of “prior restraint”, remains a cornerstone of US journalistic freedom. But leakers themselves were not protected. Ellsberg was hidden by anti-war activists while Mike Gravel, the US senator from Alaska, entered most of the leaked papers into the congressional record, and the Post played catch-up.Meanwhile Nixon, furious at the leaks, created the so-called “plumbers” covert special investigation unit, to discover if Ellsberg had further material that might affect him directly, and to discredit him. When the plumbers’ bungled break-in at the Watergate offices revealed an earlier burglary of Ellsberg’s psychiatrist’s office, the ensuing chain of scandal and cover-up eventually forced Nixon’s resignation to avoid impeachment.Ellsberg grew up the very definition of a true believer in America. Both his father, Harry, a structural engineer, and mother, Adele (nee Charsky), were the children of Russian Jewish immigrants, but had converted to Christian Science. When Daniel, born in Chicago, was six, his father found work in Detroit, building Ford’s massive Willow Run factory.Daniel won a scholarship to the elite Cranbrook school in the Detroit suburbs; a talented pianist, he practised for four to six hours a day to fulfil his mother’s dream. But in 1946, rushing to Denver for a family gathering, his father fell asleep while driving and rammed into a bridge. His mother and younger sister, Gloria, both died; Daniel recovered from his severe injuries, but ceased playing the piano.He won a scholarship to Harvard, where he studied economics, edited the college paper, and finished third in his class. Upon graduation he married a Radcliffe student, Carol Cummings, whose father was a colonel in the Marine Corps, and took up a Wilson fellowship for a year’s study at King’s College, Cambridge. In 1954, accepted as a Harvard junior fellow to pursue his doctorate, he instead joined the Marines, becoming a rare first lieutenant given command of a full company.He returned to Harvard in 1957. His dissertation, Risk, Ambiguity and Decision, contained what is now known as the Ellsberg paradox, which delineated how the preference for well-defined probabilities, over the uncertainty of ambiguity, influences decision-making, especially as it reinforces preconceived ideas. It became an important part of game theory, and Ellsberg went to work for Rand on the Department of Defense’s Command and Control research, much of which was devoted to spitballing Fail Safe/Dr Strangelove scenarios, as detailed in his 2017 book The Doomsday Machine: Confessions of a Nuclear War Planner.In 1964 he went to the Department of Defense, as special assistant for international security to McNamara’s number two, John McNaughton, before moving to the State Department and Vietnam. In 1967 he rejoined Rand to work on McNamara’s project, but was increasingly tormented by Kissinger and Nixon’s Vietnam policy; they believed that if the US opened relations with China and entered into a detente with Russia, those countries would pressure North Vietnam to come to the table while the US bombed incessantly.Ellsberg began joining anti-war campaigners, including the poet Gary Snyder, and was inspired by Randy Kehler, a draft-resister who spoke of welcoming imprisonment for his belief. Ellsberg left Washington for MIT’s Centre for International Studies a year before leaking the papers. His first marriage had ended in divorce; in 1970 he married Patricia Marx, a peace activist.In June 1971, he surrendered himself to the US attorney in Boston; asked on the courthouse steps how he felt about going to prison, Ellsberg replied: “Wouldn’t you go to prison to end this war?” He became the first civilian charged with violating the 1917 Espionage Act, and faced a maximum sentence of 115 years. The District Court judge William Byrne ruled irrelevant his public-interest defence, that the documents were “illegally classified”, and so it has been for every whistleblower since. But Byrne eventually dismissed the case because of government malfeasance, including the plumbers’ break-ins, as well as Nixon’s wiretapping of Kissinger’s aide Morton Halperin, and John Ehrlichman’s offering Byrne the directorship of the FBI.In 1974, Ellsberg’s moving interviews were a major part of the Oscar-winning Vietnam documentary Hearts and Minds. In 1978 he was awarded the Gandhi prize by Promoting Enduring Peace. In the next 40 years he was arrested around 50 times at anti-war protests. He likened the weapons of mass destruction excuse for invading Iraq in 2003 to the Gulf of Tonkin affair, and over the years supported leakers who revealed government deceptions, including Edward Snowden, Chelsea Manning and Reality Winner, who was sentenced to five years in prison for leaking a single page from an in-house National Security Agency magazine showing the NSA had concluded Russia interfered in US elections, while the government was maintaining they had not.He recognised a practical corollary to the Ellsberg paradox: the more secrets you are able to access, the less able you become to act sensibly with them. In 2021, Ellsberg released government memos from 1958, showing that the joint chiefs of staff had prepared a nuclear first-strike against Chinese bases on Quemoy and Matsu during the Taiwan Strait crisis, with a full nuclear attack planned on China should they respond. His point was that little had changed since the Pentagon Papers.Ellsberg was played by James Spader in the 2003 film The Pentagon Papers, and was the subject of a 2009 documentary, The Most Dangerous Man in America. His memoir, Secrets, appeared in 2003 and in 2021 Risk Ambiguity and Decision was updated as a book, once again challenging the concept of rational decision.Ellsberg is survived by his wife and their son, Michael, and his son, Robert, and daughter, Mary, from his first marriage. More