What happened: "In 1967...a team of analysts working for the Department of Defense prepared a highly classified study of the U.S. political and military involvement in Vietnam from the end of World War II until the present day. Daniel Ellsberg, who had served as a U.S. Marine Corps officer... had been an early supporter... and had worked on the preparation of the 1967 study. By 1969, however, Ellsberg had come to believe that the war in Vietnam was unwinnable... Some of the most damning information in the Pentagon Papers indicate that the administration of John F. Kennedy had actively helped overthrow and assassinate South Vietnamese President Ngo Dinh Diem in 1963. The report also contradicted official U.S. government pronouncements about the intensive bombing of North Vietnam, which the report stated as having no real impact on the enemy’s will to fight. In 1971... Ellsberg gave portions of the report to Neil Sheehan, a reporter at The New York Times."
Why it was so scandalous: "These published portions revealed that the presidential administrations of Harry S. Truman, Dwight D. Eisenhower, John F. Kennedy and Lyndon B. Johnson had all misled the public about the degree of U.S. involvement in Vietnam, from Truman’s decision to give military aid to France during its struggle against the communist-led Viet Minh to Johnson’s development of plans to escalate the war in Vietnam as early as 1964, even as he claimed the opposite during that year’s presidential election."