(What got Kent and Jackson State started - Invasion of Cambodia, 1970)
When Richard Nixon campaigned for President in 1968, he loudly and repeatedly asserted he had a plan to end the war in Vietnam. When news came out that we had invaded Cambodia, with no warning, no knowledge and no press on April 29th of 1970, it was largely seen as a betrayal of a pledge to end the war in Southeast Asia (they didn't call him Tricky Dick for nothing) and we were doomed to stay in an endless war forever.
It was, for all intents and purposes, the straw that broke the camel's back. We were lied to again, and it wasn't going to wash.
Read statement from The Pentagon: “The action is a necessary and effective measure to save American and other free world lives and to strengthen the Vietnamization program”.
This clip comes from the initial news reports of the invasion on the 29th. Nixon's statement would come a day later on the 30th.
But by that time the damage had been done and the series of events that led to Kent State (and Jackson State a week later), would begin their inevitable gallop towards destiny. Contrary to what revisionists may say, this was an act of anger from betrayal by people, for whom it was the majority, wanted a pointless war to end.