Answer: the bombing of Cambodia
Step-by-step explanation:
Students around the country had already being protesting the Vietnam war which President Nixon had promised he would end. Instead, in 1870 he announced that the US would invade and begin bombing Cambodia in order to target the hideouts and headquarters of the Viet Cong.
This prompted students to protest around the country including Kent State University where the students got a bit out of hand by breaking windows and setting a building on fire. In an inexcusable display of violence, some National Guards who were called, opened fire on the students and killed 4 marking a dark day in US history.