This occured the year i was born i think but it was something like this: Viet Nam war. Very unpopular at home. Many protests by anti-war groups, mostly student groups and 'hippis'. Some violent. Some peaceful. The National Guard is part of the US military. Most protesters were VERY anti military and anti government and the military was in return very anti protester. HUGE rift between two generations. At Kent State, a university, the admin called the govt on a sit in going on and they called in the National Guard. When they started trying to take kids out of the sit in things got uglier. The NG fired on peaceful but angry student protestors killing several unarmed students.
Adding to what the previous poster said, some people now think the first shot was by a very young National Guardsman who was scared and panicked, and than that's what started the whole tragedy. All in all a very ugly situation in a country that was torn apart by the Vietnam war.
Crosby, Stills, Nash & Young wrote a song about it called OHIO. The National Guard came onto the campus of Kent State and immediately started shooting indiscriminately. Four students were killed and many wounded. That's how tense it was between anti-war protesters and the police and military during the Vietnam War era.
One of the Dumbest military blunders, in this countries history. Inept leadership, arming inexperienced soldiers with live ammunition, in order to control a Student Sit-in at a college. Some students were killed and some injured, when "it was suspected", outside agitators became violent, causing things to get out of control.