He also revealed that he had defied the explicit instructions of his confidential source, whom others later identified as Daniel Ellsberg, a former Defense Department analyst who had been a contributor to the secret history while working for the Rand Corporation. In 1969, Mr. Ellsberg had illicitly copied the entire report, hoping that making it public would hasten an end to a war he had come passionately to oppose.

Contrary to what is generally believed, Mr. Ellsberg never “gave” the papers to The Times, Mr. Sheehan emphatically said. Mr. Ellsberg told Mr. Sheehan that he could read them but not make copies. So Mr. Sheehan smuggled the papers out of the apartment in Cambridge, Mass., where Mr. Ellsberg had stashed them; then he copied them illicitly, just as Mr. Ellsberg had done, and took them to The Times.

Over the next two months, he strung Mr. Ellsberg along. He told him that his editors were deliberating about how best to present the material, and he professed to have been sidetracked by other assignments. In fact, he was holed up in a hotel room in midtown Manhattan with the documents and a rapidly expanding team of Times editors and reporters working feverishly toward publication.

First, you’ll be exhilarated by some of this new information, and by having it all — so much! incredible! — suddenly available to you. But second, almost as fast, you will feel like a fool for having studied, written, talked about these subjects, criticized and analyzed decisions made by presidents for years without having known of the existence of all this information, which presidents and others had and you didn’t, and which must have influenced their decisions in ways you couldn’t even guess. In particular, you’ll feel foolish for having literally rubbed shoulders for over a decade with some officials and consultants who did have access to all this information you didn’t know about and didn’t know they had, and you’ll be stunned that they kept that secret from you so well.