On 4 May 1970, a 2000-person student demonstration at Kent State University ended in tragedy when soldiers in the Ohio National Guard fired on the crowd, killing four and wounding nine. The incident was precipitated in large part by U.S. involvement in Cambodia. President Nixon had authorized an incursion onto the Cambodian side of the Cambodian/Vietnamese border to seize Viet Cong supplies and equipment. The U.S. move into Cambodia sparked a number of protests on university campuses, including Kent State.
Richert, Lucas Paul. "Four anti-war protestors killed by National Guard at Kent State". The Literary Encyclopedia. First published 01 October 2008
[http://www.litencyc.com/php/stopics.php?rec=true&UID=3512, accessed 17 September 2014.]