It’s from Ellsberg’s book Secrets, and the setting is a meeting with Henry Kissinger in late 1968 when he was advising him about the Vietnam War. The idea of Kissinger seeking out Ellsberg for advice on Vietnam initially seems a bit unlikely, but in 1968 Ellsberg was a highly respected analyst on the war who had worked for both the Pentagon and Rand, and Kissinger was just entering the government for the first time.