The Pentagon Papers is a movie based off a series of true events involving a 47 volume research project entitled "History of U.S. Decision Making Process on Vietnam Policy." The movie tells the story of one Daniel Ellsburg, a former marine turned strategist for the U.S. government and one of the country's brightest young minds.
Ellsburg was serving at the Pentagon at the time of the Gulf of Tonkin conflict and decided to go over to Vietnam to see for himself what the situation was. What he gathered upon arrival and through a two year tenure, was that the war could not be won by the United States if things kept going at the current rate, and even then it might be a hopeless cause.
While working at RAND, Ellsburg was sought out to help with the History of U.S. project. It was because of his incredibly high security clearance that he was able to obtain a full set of the documents, all 47 volumes and over 7,000 pages of information or as he called it, "7,000 pages of lies."
Upon reading the completed Pentagon Papers, Ellsburg decided to take it upon himself to somehow get the information to the public. He tried appealing to congressmen, to no avail, and after a year of failed attempts, decided there was only one option left. According to the movie, Ellsburg released the first volume of his condensed version - 43 volumes omitting all troop positions, future plans, essentially all still usable military intelligence that could endanger the country or the stationed troops - to a reporter for the New York Times. The next day, the newspaper published the story.
This sparked a proverbial "War" about the meaning of freedom of speech and what should be considered public record. The case of New York Times v. United States went to the supreme court where they ruled in favor of the NY Times.
It is interesting to note, however, that the Supreme Court did so only due to the lack of credible evidence that the government was able to provide to get the publications stopped. The court did not remove the Espionage Act nor did it give the press freedom to publish classified documents.
The Pentagon Papers became fully declassified in 2011.