Mrs. Katharine Graham, then publisher of the Washington Post, and Executive Editor Benjamin C. Bradlee look over the Supreme Court decision that allowed Post to publish stories based on the Pentagon Papers, on June 30, 1971 (CHARLES DEL VECCHIO/TWP)

The disclosure of the Pentagon Papers four decades ago stands as one of the most significant leaks of classified material in American history. Ever since, in the eyes of the government, the voluminous record of U.S. involvement in Vietnam has remained something else: classified.

In the Byzantine realm of government record-keeping, publication of a document in the country’s biggest newspapers, including this one, does not mean declassification. Despite the release of multiple versions of the Pentagon Papers, no complete, fully unredacted text has ever been publicly disclosed.

On Monday, the National Archives and Records Administration will change that, as it officially declassifies the papers 40 years to the day after portions were first disclosed by the New York Times. In doing so, and in making the papers available online, the Archives could provide researchers with a more holistic way of understanding a remarkable chapter of U.S. history.

It could also bring a small measure of solace to advocates of open government frustrated by what they see as the overzealous classification of important documents. They note that tens of thousands of the classified diplomatic cables released by WikiLeaks also remain classified.

“The fact that the Pentagon Papers were still secret is an embarrassment to the United States government,” said John Prados, a senior fellow at George Washington University’s National Security Archive, a nonprofit research organization. “You’ve been able to read them for 40 years, but they’re still secret.”

While the complete version runs to approximately 7,000 pages, the set leaked to the Times by defense analyst Daniel Ellsberg had pages missing and lines that were made illegible during photocopying. (Ellsberg had to lug the volumes in batches to the office of a colleague’s girlfriend and, once there, used a copy machine that could scan only one page every few seconds.) Other versions were either heavily redacted or simply incomplete.

It’s not clear how many secrets remain within the documents being released Monday.

There might be small surprises lurking within, including the names of those involved in the project who have not been previously identified. But participants who are already known have reacted to the announcement of the declassification mostly with a shrug.

“I had almost forgotten about them,” said Leslie H. Gelb, who headed the task force that wrote the report and is now president emeritus of the Council on Foreign Relations. Retired Army Gen. Paul Gorman, a senior military officer who worked alongside Gelb, said, “I haven’t given them a thought in 10 years or more.”

Nonetheless, Gelb and others say that the documents themselves still have lessons to teach about government and conflict. The Pentagon Papers were created by an administration attempting to quietly rethink the quagmire that was the Vietnam War. But their publication in 1971, at a time when the American public had largely turned against the war, was explosive because it revealed a startling gulf between the optimistic public statements of the nation’s top leaders and their increasingly grave private doubts.

“This was a secret history project to try to figure out why we were in such a national security tangle. And now with all the material together in one place, you can see how our government wrestled with the problem,” said Timothy Naftali, the director of the Nixon Presidential Library and Museum, one of three presidential libraries that has a complete classified set of the Pentagon Papers.

While Prados, of the National Security Archive, said he lodged multiple and apparently futile requests for declassification reviews over the years, government officials suggested the reason for the delay was, more than anything, a reflection of bureaucracy. In late 2009, when President Obama established the National Declassification Center to help streamline review procedures, there was an estimated backlog of 400 million pages that needed to be processed.

At the time, officials wanted to identify specific projects of significant interest to prioritize. And with the 40th anniversary of the leak of the Pentagon Papers approaching, the documents — known officially as the Report of the OSD (Office of Secretary of Defense) Vietnam Task Force — were natural candidates, said A.J. Daverede, chief of the production division of the NDC and head of the project to declassify the Pentagon Papers.

Ellsberg, who wrote a memoir about the Vietnam era and his decision and remains a fierce critic of government secrecy, said he sees the declassification of the Pentagon Papers as a “nonevent.”

But if there’s something to be taken away from the declassification, Ellsberg said, it’s that the public should begin to question the “absurd” mystique of secrecy and government classification.

Declassification, he said, “might encourage people to really look at what it means that they’ve held this back so long. It should lead them to ask what else they’ve held back wrongly.”

Staff researcher Julie Tate contributed to this report.