An excerpt from my dissertation:

President Nixon revealed the US invasion of Cambodia on April 30, 1970, in a televised speech. At a news conference the next day National Student Association president Charles Palmer, flanked by ten collegiate student body presidents, denounced the invasion and Nixon’s “odious disregard of the constitution” and called for his impeachment. 

The nation’s first student strikes in response to the invasion had already been called by the time Palmer spoke, and by Monday walkouts had begun, with NSA’s enthusiastic support, at dozens of campuses. Throughout the weekend NSA staff worked with an impromptu national strike center at Brandeis University to coordinate, encourage, and publicize strike activity as best they could.

Many campuses closed as the protests escalated, but Kent State in Ohio stayed open, and that state’s governor — facing a deteriorating situation on campus in the final stages of a tight re-election race — called out the National Guard. On Monday, a little after noon, Guard troops on the campus fired on a crowd of protesters. The gunfire killed four people, including two students who were walking past the protest on their way to class.

This was not the first time, or even the first time in recent years, that American students had been killed by agents of the government in the course of a campus protest. In early 1968 police had fired on anti-segregation activists at South Carolina State University, killing three. And it would not be the last — nine days after Kent State, two students at Jackson State College in Mississippi were killed in circumstances similar to those of the South Carolina shootings.

But unlike in South Carolina and Mississippi, the students killed at Kent State were white. And crucially, the Kent State killings were documented on film — a Kent State photography major took two rolls of photos of that day’s protest and its aftermath, and his photographs went out over the AP wire that night. One image — of a young woman kneeling over the body of one of the dead, screaming with arms outstretched — appeared on the front pages of newspapers all over the country the next day. The Kent State killings unleashed an unprecedented wave of protest, forcing hundreds of campuses to close for the semester.