It's hard to imagine an American president in this intensely security-conscious age leaving the White House in the middle of the night to meet protesters on their turf.

It happened in May 1970. President Richard Nixon was under intense criticism for widening the Vietnam War to Cambodia. Four Kent State University students had been killed by National Guardsmen just days before. Thousands of young protesters quickly mobilized and headed to Washington, D.C.

Around 4:00 a.m. on May 9, Mr. Nixon abruptly decided to surprise a group gathered at the Lincoln Memorial. The Nixon Presidential Library and Museum has released a series of recordings, including dictation from President Nixon to his chief staff, H.R. Haldeman, describing his version of that night's events....