The country is currently not the closest we’ve ever come to nuclear war, says Daniel Ellsberg, but we are still likely to face annihilation. And recent false alarms in Hawaii and Japan, erroneously alerting residents to incoming missiles, do nothing to quell the anxiety. “This was a little rehearsal,” Ellsberg says, “but not the first one, of being right on the edge of destruction.”

The history-making whistleblower softens no blows, sugarcoats nothing. The 86 year-old went to battle with the U.S. government when he released the Pentagon Papers in 1971, walking away a free man after a landmark trial in which he was charged under the Espionage Act. But Ellsberg has more alarms to sound. “The threats to exterminate North Korea for acts by its leaders are illegal, immoral, monstrous,” he tells TIME from his home in Kensington, Calif. There is a chance humanity will survive the weapons of its own making — but “it’s very unlikely.”

Ellsberg’s urgent interest in nuclear debate runs deep. Before he released the material that revealed the government misled the public about the extent of U.S. involvement in the Vietnam War — a story newly refreshed by Matthew Rhys as Ellsberg in Steven Spielberg’s latest, The Post — Ellsberg was a nuclear analyst and consultant to the Department of Defense, where he helped draft Secretary Robert McNamara’s plans for nuclear war. In December, he released The Doomsday Machine: Confessions of a Nuclear War Planner, a book which unravels his untold history with 1960s nuclear secrets and his would-be plans to release them.

The book — and the first-hand understanding of the dangers of nuclear war it yields — come at an essential moment, as President Donald Trump knocks on the door of conflict with North Korea. The claim that casualties resulting from war with North Korea would be “over there” rather than “over here,” put forth by Republican Sen. Lindsey Graham of South Carolina in August, is both “vile” and “false,” Ellsberg declares; Kim Jong Un has planned for the ability to cause harm in the U.S., of that we can be sure.

“Trump could not totally eliminate, even in a surprise nuclear attack, the North Korean ability to retaliate,” Ellsberg says. And don’t forget chemical warfare capability: Kim’s half-brother Kim Jong Nam was assassinated just under a year ago with the nerve agent VX. The risk of conflict with North Korea, Ellsberg contends, is likely to be far, far greater than what we’ve been told. “If I had access to those official estimates right now, I would certainly consider putting that information out to the press, to the public, to Congress and the world,” he says, adding a plea to those who might have truths within their reach: “Don’t do what I did; don’t wait until the bombs are falling or thousands have died if you have information that might avert that.” Also on Ellsberg’s whistleblowing wish list: the 6,000-page 2012 Senate report on torture.

The decision whether to leak or keep nuclear secrets was made for him. The most intriguing tale in his book, fodder for a political thriller on its own, involves the documents he never made public. All the while Ellsberg busily copied 7,000 classified pages from the “History of U.S. Decision-Making in Vietnam, 1945-68” report, he also copied a trove of documents on the country’s nuclear program—pages he judged even more crucial, more imperative to the fate of the country than those on the war in Southeast Asia. Pages he always intended to release at the conclusion of his Pentagon Papers trial. But Mother Nature denied him the chance: he gave the documents to his brother Harry, who held them in his Westchester County, N.Y. basement for two years, moved them to his compost heap when the FBI started hunting for Ellsberg in 1971, then—the day before mysterious men were seen prodding the spot where they’d been in his yard—to a place near a junked gas stove in the local dump. But a storm swept the papers away, never to be discovered again.

Ellsberg is staunchly in favor of counter-proliferation. “It’s possible to make a case for a small deterrent force of nuclear weapons if your neighbors or adversaries, opponents at war, have such weapons,” Ellsberg admits — but he cites Herbert York, the nuclear physicist who once ran the Lawrence Livermore Laboratory, when he adds that we can justify closer to one than 100 weapons. “You do not need the ability to cause nuclear winter to achieve any national objective, like deterring nuclear attack or occupation—you do not need the ability to end most life on earth,” he says. And yet we have it, as does Russia. On the plus side, he adds: North Korea does not.

Ellsberg believes there is a real possibility that the U.S. will go to war with North Korea, and that such a war would be catastrophic. It would not bring about the end of humanity as a war with Russia has the potential to do, he says, but it would kill millions and risk putting the country on a dangerous course, particularly under the leadership of President Trump. Ellsberg speculates there would be a “crackdown” in the U.S., in the event of North Korean retaliation. He envisions a suspension of the Constitution, a country with detention camps, mass deportations and total surveillance — the end of democracy.

If there was one lesson the whistleblower learned in the Pentagon Papers episode that we must apply now, it’s this: “Intelligent, patriotic, conscientious men can in secret pursue monstrous policies,” he says. “In other words, in the darkness, prevented from any public awareness, very smart men can act not only stupidly, but also crazily.”

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Write to Lucy Feldman at lucy.feldman@time.com.