John Prados, an independent scholar who pried reams of classified documents from government vaults, publishing his findings in more than two dozens books that broadened and sometimes challenged the known history of World War II, the Vietnam War, the Cold War and the CIA, died Nov. 29 at a hospital in Silver Spring, Md. He was 71.

The cause was cancer, said his partner, Ellen Pinzur. Prados' death was announced by the National Security Archive, a research institute at George Washington University where he had been a senior fellow for 25 years. The organization described him in a tribute as "the ultimate prospector in the gullies of the documentary gold rush," ever in search of treasure yet to be mined from U.S. government archives.

Prados came of professional age amid the ferment of the 1970s, when revelations of government scandal and subterfuge caused many Americans to question their leaders in ways they never had before.

He was a history student at Columbia University when the Watergate affair began to unfold, ultimately ensnaring President Richard M. Nixon and driving him from office. Prados completed his first graduate degree in 1975, the year Saigon fell to communist forces, marking the end of the Vietnam War, in which nearly 60,000 American troops lost their lives.

An antiwar activist during those years, Prados devoted his scholarly career to ferreting out government secrets, he once told the New York Times, because "the American people not only have a need but a right to know their history."

Tom Blanton, the director of the National Security Archive, observed in an interview that Prados "almost invented and certainly personified the identity of an independent scholar." He held no official title until Blanton persuaded him to join the institution in 1997.

In his scholarly work, Prados explored the Pacific theater of World War II in the volume "Combined Fleet Decoded" (1995) and the European theater in "Normandy Crucible" (2011).

One of his best known volumes was "Vietnam: The History of an Unwinnable War, 1945-1975" (2009), offering what historian Arthur Herman, reviewing the book in the Wall Street Journal, described as "a detailed picture of a U.S. government unwilling to confront its mistakes and an American military baffled by a guerrilla insurgency."

Prados developed a particular expertise in intelligence matters, examining the National Security Council in "Keepers of the Keys" (1991) and the CIA in volumes including "Safe for Democracy: The Secret Wars of the CIA" (2006) and "The Ghosts of Langley: Into the CIA's Heart of Darkness" (2017).

Reviewing the minutes of a secret NSC meeting in 1975, he discovered Secretary of State and national security adviser Henry Kissinger grousing to President Gerald Ford that "it is an act of insanity and national humiliation to have a law prohibiting the President from ordering assassination."

Prados relied heavily on the Freedom of Information Act, a 1967 law that allows citizens to request access to government documents, and displayed by all accounts heroic stamina in reviewing the mountains of material that such requests often yield. Celebrated among many anti-secrecy activists, he was also an occasional annoyance to government officials.

As the National Archives prepared to formally release the Pentagon Papers in 2011, four decades after the 7,000-page secret history of the Vietnam War was first published by newspapers including the New York Times and The Post, federal officials initially announced that 11 words would be redacted from the release.

They reversed course after realizing that the passage in question had already appeared in a version of the papers released years earlier, and that any redaction would only draw attention to whatever matter they wished to conceal.

In an internal email at the time, a declassification official remarked that Prados had been the researcher "most aggressive in pursuing" the Pentagon Papers and predicted that he would "likely scope out the 'declassified' page very quickly" and "parade this discovery like a politician on the 4th of July." In the end, the Pentagon Papers were released with no redaction, leaving readers —including Prados — to guess at the identity of the 11 words.

Prados wagered that the contents had something to do with electronic surveillance in the Gulf of Tonkin in February 1965. But in any case, "the most distressing aspect of the 11 words episode," he told the Times, "is that anyone at all felt a need to try and put toothpaste back into the tube after the passage of four decades' time."

John Frederick Prados was born in Queens on Jan. 9, 1951. When he was in middle school, his family moved to Puerto Rico, where his father was born, and where he managed an athletic stadium. Prados' mother was from Kansas and taught English as another language.

His father had served in the military, and Prados at one point aspired to attend the U.S. Military Academy at West Point. "War games and my whole interest in the subject is a sublimation of that," he told The Post.

Instead, he entered Columbia University, where he received a bachelor's degree in 1973. He remained at Columbia for his graduate studies, receiving his first master's degree in 1975, his second master's in 1977 and a doctorate in 1982, all in political science.

Prados sought to calculate the extent of Soviet strategic forces during the Cold War in his dissertation, which became his first book, "The Soviet Estimate" (1982). He returned to the contest between the superpowers in volumes including "How the Cold War Ended" (2011).

Prados' marriage to Jill Gay ended in divorce. Survivors include his partner of 25 years, Ellen Pinzur of Silver Spring; two daughters from his marriage, Dani Prados of Granite Falls, Minn., and Tasha Prados of Takoma Park, Md.; a brother; and a sister.

In the early years of his career, Prados earned much of his livelihood designing board games that pitted players against each other amid historical events including the Napoleonic wars, the Civil War, World War II and the Vietnam War. His board game "Rise and Decline of the Third Reich," released in 1974, became one of the most popular strategy contests of its kind.

Although they might have seemed like separate ventures — one grounded in history, the other an invitation to alternative history — Prados saw his archival work and the design of his board games as inextricably linked.

With his games, he said, he had no interest in "breeding militarism," he remarked. Their purpose, rather, was to reveal at the safety of a gaming table "the difficulty of conducting war" — and its "horrendous costs."

The Washington Post's Nate Jones contributed to this report.