Today in History - May 4, 1970 - Four Student Protesters Killed at Kent State

It all began when the Nixon Administration decided to invade Cambodia.... According to Wikipedia:

Not all of the members of the administration agreed that an invasion of Cambodia was either militarily or politically expedient. Secretary of Defense Melvin R. Laird and Secretary Rogers were both opposed to any such operation due to their belief that it would engender intense domestic opposition in the U.S. and that it might possibly derail the ongoing peace negotiations in Paris (they had both opposed the Menu bombings for the same reasons).[24] Both were castigated by Henry Kissinger for their "bureaucratic foot-dragging."[25] As a result, Laird was bypassed by the Joint Chiefs in advising the White House on planning and preparations for the Cambodian operation.[26] 

On the evening of 25 April Nixon dined with his friend Bebe Rebozo and Kissinger. Afterward, they screened one of Nixon's favorite movies, Patton, a biographical portrayal of controversial General George S. Patton, Jr., which he had seen five times previously. Kissinger later commented that "When he was pressed to the wall, his [Nixon's] romantic streak surfaced and he would see himself as a beleaguered military commander in the tradition of Patton."[20] The following evening, Nixon decided that "We would go for broke" and gave his authorization for the incursion.[20] The joint U.S./ARVN campaign would begin on 1 May with the stated goals of: reducing allied casualties in South Vietnam; assuring the continued withdrawal of U.S. forces; and enhancing the U.S./Saigon government position at the peace negotiations in Paris.[27] In order to keep the campaign as low-key as possible, General Abrams had suggested that the commencement of the incursion be routinely announced from Saigon. At 21:00 on 30 April, however, President Nixon appeared on all three U.S. television networks to announce that "It is not our power but our will and character that is being tested tonight" and that "the time has come for action." He announced his decision to launch American forces into Cambodia with the special objective of capturing "the headquarters of the entire communist military operation in South Vietnam."[28] Full Article Regarding the Cambodia Campaign

Nationwide this led to a series of Anti-War protests and a photographic image that was burned into the collective retinas of America when on May 4th.....(From History.com)

In Kent, Ohio, 28 National Guardsmen fire their weapons at a group of antiwar demonstrators on the Kent State University campus, killing four students, wounding eight, and permanently paralyzing another. Two days earlier, the National Guard troops were called to Kent to suppress students rioting in protest of the Vietnam War and the U.S. invasion of Cambodia. The next day, scattered protests were dispersed by tear gas, and on May 4 class resumed at Kent State University. By noon that day, despite a ban on rallies, some 2,000 people had assembled on the campus. National Guard troops arrived and ordered the crowd to disperse, fired tear gas, and advanced against the students with bayonets fixed on their rifles. Some of the protesters, refusing to yield, responded by throwing rocks and verbally taunting the troops.Full Post

The musical response came from Crosby, Stills, Nash and Young......