Surprised I had never heard of this incident before. Incredibly well written article and some fascinating footage from a bygone..yet somehow familiar, era.
At the risk of sounding like that guy who thinks everybody should know about the same things he does, I am admittedly pretty shocked to hear somebody on HN (who is also perhaps a US citizen?) didn't know about this. That's not intended as a commentary on you, but rather on our society and the potential for even enormous events to fall out of the common social consciousness.
I'm not surprised at all, unless you're old enough to remember it when it happened. (I am.) We do a poor job of teaching American history. My high school American history class didn't spend a lot of time on the Vietnam war -- basically we were told it was an unpopular war and there were protests. For example, I don't remember any mention of the My Lai massacre or the Tet Offensive. I did not take a general American history class in college; but, I doubt it would have covered Vietnam for more than a day or two.
Do you maybe at least recognize the photo at the top of the Wiki article? https://en.wikipedia.org/wiki/Kent_State_shootings