Daniel Ellsberg speaks at a 2013 rally in support of Army Pfc. Bradley Manning — now the recently pardoned leaker Chelsea Manning — outside the gates of Fort Meade, Md. (Patrick Semansky/Associated Press)

The most dangerous man in America is asking to borrow my scarf.

I’ve known Daniel Ellsberg for only five minutes, but, curious, I unwind it from my neck and give it over. One-handed, with a flick of his wrist, the famous Pentagon Papers whistleblower produces an elegant knot. With another flick, the knot disappears.

Not a bad feat, though it hardly measures up to his copying and leaking thousands of pages of classified documents on the Vietnam War to the New York Times — an act that eventually changed the course of history.

Henry Kissinger, Richard Nixon’s national security adviser and later secretary of state, dubbed Ellsberg “the most dangerous man in America,” which became the title of an award-winning 2009 documentary.

Almost five decades after the first Pentagon Papers story was published in 1971, revealing the secret history of the Vietnam War, the 85-year-old Ellsberg still isn’t done making trouble. That was clear on a Georgetown University stage earlier this month, shortly after the scarf encounter.

“Something like the Pentagon Papers should be coming out several times a year,” Ellsberg told journalist and scholar Sanford Ungar, who organized the two-day symposium, “Free Speech Legacies: The Pentagon Papers Revisited.”

If Ellsberg had had access to the Senate Intelligence Committee report on CIA torture, a summary of which was released in 2014, “I would have put that out,” he said.

There’s plenty more, he’s sure.

“The secrecy system operates overwhelmingly to keep important information from the American public,” he said.

Whistleblowers are the best defense, he believes — but there aren’t enough of them.

An admirer of two other major leakers, Chelsea Manning and Edward Snowden, Ellsberg wants more.

“Is three whistleblowers of this scale about right in 45 years?” he demanded.

He knows, though, that they have paid a big price — and the legal troubles of other Obama-era leakers, such as Thomas Drake and John Kiriakou, underscore his point.

Manning, a former Army intelligence analyst, leaked a huge tranche of classified information — including a video showing an American airstrike killing Iraqi civilians — through WikiLeaks. Court-martialed, the transgender woman formerly known as Bradley Manning went to prison for seven years; President Barack Obama commuted her sentence in his final days in office.

Snowden, a former National Security Agency contractor who revealed shockingly widespread electronic surveillance of American citizens by their government, will never return to the United States, Ellsberg said. Exiled in Russia, he would not be allowed to explain his motivations during trial because he is charged under the Espionage Act, which allows no public-interest defense.

Ellsberg entertained the Georgetown crowd with spot-on impressions of Nixon and Kissinger, and tales about failing to master Twitter and digital encryption.

“I had to rely on Xerox — I used the cutting-edge technology of my day,” he quipped.

The government case against him ended in a mistrial, sparing him what he expected would be life in prison.

Now, with President Trump threatening to prosecute government leakers, he said, “we’re coming full circle.”

“We’re back with Nixon, as we have been all along.” All presidents lie, Ellsberg said — and both Nixon and Trump have stated that when the president does something, it is, by definition, legal.

When Nixon said it to TV interviewer David Frost, he was referring to government agents’ break-in at Ellsberg’s psychiatrist’s office — an effort to find material to blackmail him.

That crime, top Nixon aide John Ehrlichman later said, was “the seminal Watergate episode” — the original sin leading to Nixon’s eventual demise.

But Ellsberg said that “the things that were crimes under Nixon are no longer crimes,” after post-9/11 Patriot Act legislation.

“Even killing people is something Obama has proclaimed the right to do,” he said, referring to Anwar al-Awlaki, an American citizen and radical Islamic cleric assassinated by a CIA drone strike in Yemen.

Ellsberg thinks Trump — whose associates are already under FBI investigation for Russian connections — will avoid Nixon’s fate.

“If he were facing a Democratic Congress, he’d be in great trouble. If he were facing a Republican Congress that had any principle, any conscience, any shame . . . but he doesn’t have that,” Ellsberg said. “It won’t be a problem. And I’m sorry to say that.”

His own leak didn’t accomplish its purpose, he said.

“The Pentagon Papers didn’t shorten the war by a day,” he said. But Ellsberg’s leak did reveal the government’s longtime cynicism about the war: that President Lyndon Johnson had believed it was unwinnable, even as more bombs fell and as more soldiers and civilians died.

What’s more, it established an important press rights precedent: that the government can’t use “prior restraint” to prevent publication, which Nixon tried and failed to do when he attempted to enjoin the Times and The Washington Post from publishing the papers.

Ellsberg stands by what he did — just as he fully approves of Snowden and Manning because they brought light to government deception and malfeasance.

Despite the threats that such leakers will endanger national security and have “blood on their hands,” he said, no such harm has been proved.

Now it’s time to bring more to light.

“I would like others, like Snowden, to think about their oath to the Constitution and whether they are obeying it” by keeping silent, he said.

He offered another subversive thought.

“Manning and Snowden and I all thought the same words, which I heard them say: ‘No one else was going to do it, someone had to do it — so I did it.’ ”

For more by Margaret Sullivan visit wapo.st/sullivan