By 1967, Secretary of Defense Robert McNamara was becoming very pessimistic about the Vietnam War. He no longer believed it could be won at acceptable cost.

Tom Winship, editor of the Boston Globe, holds the June 1971 edition that carried excerpts from the secret Pentagon report on Vietnam. AP/WIDE WORLD PHOTOS

American casualties were growing, and public support for the war was getting shaky. He created a task force in the Defense Department to study how the United States had gotten into this fix. In 1969 the task force completed a top-secret history of U.S. policy from 1945 through early 1968, accompanied by thousands of pages of classified documents. Officially titled United States-Vietnam Relations, 1945–1967, it is usually called the Pentagon Papers.

Daniel Ellsberg had spent more than a year in Vietnam as a roving investigator for the U.S. government. What he saw convinced him that U.S. policy...