New York Times reporters, editors, and copyboys secretly gathered in a room at the Hilton Hotel in Manhattan. Inside, a Xerox machine churned out copies of a top-secret study titled United States—Vietnam Relations, 1945–1967: A Study Prepared by the Department of Defense that stretched thousands of pages. While the photocopy machine worked overtime to fill two five-foot-tall filing cabinets with copies of the report, the journalists worked to fill in the blanks of their own copy, knowing that no American paper had written a series of articles based on a still-classified document before. The Times’ investigation into the study that came to be known as the Pentagon Papers was the first; the legal battle that followed ensured it wouldn’t be the last.

Everything about the Pentagon Papers had been shrouded in secrecy. Secretary of Defense Robert McNamara tasked 36 defense department analysts with authoring the study in June 1967. They were to use existing documents to write a comprehensive history of the Vietnam War up to that point; nobody outside the team was consulted for the study. When they finished in January 1969, the analysts had written 3,000 pages and connected another 4,000 pages of reference documents, leading to a behemoth of a study that covered more than 20 years of conflict. Within those 7,000 pages was the truth about why the United States joined the conflict in Vietnam.

The Vietnam War was part of the United States’ plan to stop the spread of communism and temper China’s influence. Yet the conflict’s purpose was hidden from American citizens, many of whom opposed the war, and its official goal was to prevent the communist North Vietnam from swallowing up South Vietnam. Many aspects of the Vietnam War reflected this focus on secrecy: massacres were covered up; peace talks between the United States and North Vietnam were kept quiet; and the conflict’s expansion to neighboring Cambodia and Laos was kept out of the headlines. It was a shadow war that would claim the lives of 58,315 Americans and injure hundreds of thousands more. Meanwhile, 2 million civilians on both sides of the conflict died, as well as 250,000 South Vietnamese soldiers and 1.1 million fighters from the north.

The Pentagon Papers brought many of those secrets to light. It showed the US government’s “massive mismanagement” of the Vietnam War, as former Secretary of State Henry Kissinger would later describe it. The study was a 7,000-page-long poke in the eye for the two administrations whose actions it described, and would surely cause a headache for whomever was in the White House when it was published. Yet there was little chance of that happening. Only 15 copies of the 47 volume report were created, and most were kept within the government’s control. The stories detailed within the Pentagon Papers were never intended for public consumption.

That’s where Daniel Ellsberg and Anthony Russo came in. Ellsberg was a former Marine who worked in the Pentagon before spending two years in South Vietnam. Russo had been an engineer at NASA. Now they both worked for RAND Corporation, a policy group that works closely with the American government, and both came to oppose the Vietnam War around the time work started on the Pentagon Papers. Ellsberg’s opposition rose after he visited a number of anti-war protests; Russo’s came after he spent years interviewing Vietnamese soldiers tortured by the CIA. They decided the information within the Pentagon Papers had to get out.

They devised a plan to release the study. It was quintessentially 1970s: sections of the report were removed from a RAND Corporation safe at night, taken to a small advertising agency nearby, and Xeroxed until morning came. Ellsberg then leaked copies of the massive report to the Times, the Washington Post, and other news outlets. Everything was done with the same secrecy as the original study: few knew of Ellsberg’s plans, the Times journalists operated from that room at the Hilton, and even the typesetting of the first article based on the Pentagon Papers was handled in secret before it was sent to the printer for the front page of the June 13, 1971 edition of the Times.

President Richard M. Nixon was outraged when he read the story. Up to then, he didn’t even know the Pentagon Papers existed, much less they had been leaked. Nixon initially focused on the “treasonable action” of whoever leaked the study before deciding that the newspaper had become his administration’s number one enemy.

The following day, Attorney General John Mitchell sent a telegram to Times publisher Arthur Ochs Sulzberger. Mitchell claimed that reporting on the Pentagon Papers was “directly prohibited by the provisions of the Espionage Law.” He added: “Accordingly, I respectfully request that you publish no further information of this character and advise me that you have made arrangements for the return of these documents to the Department of Defense.”

Sulzberger rejected the government’s demand, and the Nixon administration responded with a federal court injunction for “prior restraint” to prevent the paper from publishing more stories, which the Times appealed. The Washington Post published its own reports on the Pentagon Papers starting five days after the Times’ first piece. Nixon’s administration also sought an injunction on the Post, and when it was denied by a federal judge, appealed that decision. Eventually the cases made their way to the Supreme Court, which agreed to hear them jointly on June 26, just 13 days after the first story on the Pentagon Papers had been published.

The Times argued in its brief that prior restraint, forcing the paper to cease any future publication involving the Pentagon Papers, equated to censorship. It would have been one thing if the Nixon administration had issued criminal sanctions for stories that had already been published, but silencing the press before a story made its way to the presses was another matter. The Post said the government failed to prove that the Pentagon Papers reporting posed a threat to national security, which meant it couldn’t justify censoring the press.

The justices needed just four days to rule 6-3 in favor of the Times and the Post.

Justice Hugo Black wrote in his opinion:

This ruling has provided legal cover to American-based media outlets publishing government secrets ever since. The laws in the United Kingdom are different. It’s why in 2013 the British government could force The Guardian to destroy hard drives containing classified information former NSA contractor Edward Snowden provided the newspaper. The New York Times and Washington Post Post—both of which received the same set of Snowden documents as the Guardian—didn’t have the same problem. As with the Pentagon Papers, they had the First Amendment on their side.

This, unfortunately, has not prevented government from hounding journalists for publishing its secrets. It merely changed tactics, going after individual reporters rather than the outlets themselves. Much more recently, Associated Press journalists, the Times’ James Risen, and Fox News correspondent James Rosen were targeted during leak investigations because they refused to give up their sources. Government officials still sometimes attempt to prevent the publication of stories under the guise of protecting national security, even if they cannot back up such claims and they usually lose.

Whistleblowers have also been targeted. Ellsberg and Russo were charged under the Espionage Act for leaking the Pentagon Papers. The case ended in a mistrial after the Nixon administration illegally wiretapped Ellsberg, broke into his psychiatrist’s office to steal documents, bribed the trial judge with the possibility of becoming the director of the FBI, and otherwise bungled the case. Yet the law remained unchanged, and under President Barack Obama the department of justice pursued more prosecutions under the Espionage Act (eight) than every previous administration combined.

The government celebrated the 40th anniversary of the Pentagon Papers on June 13, 2011 by making the full text available to the public for the first time. Ellsberg had previously omitted some aspects of the report, including peace talks with North Vietnam, from the copies he shared with the press. (He feared their publication would derail negotiations and extend the war he was trying to stop.) The Times and the Post didn’t publish the Papers in full; nor did the Beacon Press in a book version of the supposedly complete report. Other versions published by the government had been redacted.

Finally, 40 years later, the Pentagon Papers debuted in its entirety.