May 4, 1970: 4 students killed by National Guard at Kent State

Ohio National Guardsmen opened fire on anti-war protesters at Kent State University in Ohio, killing four students and wounding nine others who were protesting the US invasion of Cambodia.  Many historians point to this killing, and the less known shootings at Jackson State later that month, as the incident that changed anti-war sentiment from a fringe group primarily led by young people into a broad-based movement.  

Neil Young's "Ohio," performed by Crosby, Stills, Nash and Young, capsulizes the anger of the day.