Which 1971 document revealed the US government's pessimistic assessment of the war?

The Pentagon Papers were the 1971 document that revealed the U.S. government’s pessimistic assessment of the Vietnam War.

Officially titled the Report of the Office of the Secretary of Defense Vietnam Task Force, the study was commissioned by Defense Secretary Robert McNamara in 1967. It examined U.S. political and military involvement in Vietnam from 1945 through 1968 and ran to 47 volumes, including thousands of pages of analysis and original documents.

Daniel Ellsberg, a former Pentagon analyst who had worked on the study, copied portions and gave them to journalist Neil Sheehan. The New York Times began publishing excerpts on June 13, 1971. The documents showed that successive administrations had expanded U.S. involvement and privately doubted claims made publicly about progress and the war’s likely outcome.

A common mix-up is treating The New York Times leak as the document itself. The newspaper published the material, but the document was the Pentagon study. The Supreme Court’s decision allowing continued publication became a major press-freedom milestone.