In 1971, Daniel Ellsberg decided to make available to the New York Times (and then to other newspapers) 43 volumes of the Pentagon Papers, the top- secret study prepared for the Department of Defense examining how and why the United States had become embroiled in the Vietnam conflict. But he made another critical decision as well. That was to keep confidential the remaining four volumes of the study describing the diplomatic efforts of the United States to resolve the war.

Not at all coincidentally,...