The "most dangerous man" in the United States died at the age of 92 on Friday, leaving behind him a long legacy of exposing Washington's policies during its war on Vietnam.

The title in question was given to Daniel Ellsberg by infamous US diplomat Henry Kissinger.

A military analyst, historian, and journalist, Ellsberg was responsible for leaking highly classified internal Pentagon documents, known as the Pentagon Papers or officially titled Report of the Office of the Secretary of Defense Vietnam Task Force, in 1971.

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The 7,000-page-long papers issued by the Department of Defense recorded secrets of the US political and military involvement in Vietnam from 1945 to 1967, shedding light on how US top officials were deceiving the American public about why their sons are dying in Vietnam.

 A controversial person who once fought in Vietnam and later turned into a peace activist, Ellsberg was considered by some a hero and by others a traitor, nevertheless it remained true that Ellsberg made a major impact on the United State's politics.

“He had concluded the violence in Vietnam was senseless and therefore immoral. His conscience told him he had to stop the war,” Neil Sheehan, the journalist who was the first to publish parts of the exposed documents, wrote in “A Bright Shining Lie: John Paul Vann and America in Vietnam.”

Read more: US more polarized today than during Vietnam War: Kissinger

“Ellsberg, in whatever incarnation and in any job, was no ordinary man,” David Halberstam wrote in his 1979 book “The Powers That Be.”

“He was an obsessive man. That which he saw, others must see, that which he believed, others must believe. Thus as he became increasingly disillusioned, he also became a force. No one entered an argument with him lightly or left it exactly the same,” Halberstam added.

Former US President Lyndon Johnson made vows during the 1960s not to send America's sons to die in wars in Asia. 

After Ellsberg joined the Pentagon in 1964 as a strategic analyst, he was tasked to work on a project to increase US presence in Vietnam. During his Vietnam deployment, the journalist saw firsthand what the United States military was doing with Vietnamese civilians.

“Nothing else,” Ellsberg wrote later, “seemed so purely incomprehensibly evil as the deliberate bombing of women and children.”

Read more: US bombing in Vietnam War killing Laotian people to date

We were the wrong side

Halberstam wrote that he [Ellsberg] “became fascinated by the question of war crimes.”

After his return from Vietnam, the now-reformed Ellsberg tried to explain the mess of the war to then-National Security Advisor Walt Rostow, but he was brushed off by Rostow and told “You don’t understand,” and "victory is near".

“It wasn’t that we were on the wrong side,” Ellsberg said later. “We were the wrong side.”

It was through his job at the RAND Corporation that Ellsberg gained access to the documents, where he was able to sneak copies of the Pentagon Papers.

The New York Times began on June 13, 1971 to publish parts of the vast material handed to them by Ellsberg. Nixon biographer John Farrell called that day as “the Sunday morning that sired the flames that came to claim his presidency.” Washington then barred the NYT from further publishing the material, however, Ellsberg had already sent copies to other major newspapers.

Enemy of the government

His efforts were not recognized by some others as anti-war, but as anti-government during a time of war.

"The press should be able to fulfill its secular role of exposing rascals and mistakes in government without making common cause with the enemies of government,” General Maxwell Taylor, retired head of the Joint Chiefs of Staff, wrote in a New York Times op-ed then.

As the DoJ went after Ellsberg to put him on trial, Nixon also ordered his officials to gather information to discredit him. “We’ve got to get him,” the president demanded then.

After the full Pentagon Papers were declassified and released in 2011, the anti-war journalist called on others to proceed to pursue and expose the US secrets regarding the wars in the Middle East.

“The personal risks are great,” Ellsberg wrote in The Guardian. “But a war’s worth of lives might be saved.”

For decades, whenever a whistleblower leaked secrets, Ellsberg would invariably be asked to comment. “I think he’s done an enormous service, incalculable service.”

Ellsberg praised NSA whistleblower Edward Snowden who leaked America's spy practices against its citizens in June 2013. “I think he’s done an enormous service, incalculable service."

Read more: Snowden, from exile to The Guardian: No regrets

“It can’t be overestimated to this democracy. It gives us a chance,” he added.

“His memoir, ‘Secrets,’ should be read in every American history class as a primer on the war in Vietnam,” anti-war activist Mark Rudd said.