What did Neil Young write about in the song Ohio?

Neil Young wrote "Ohio" in response to the Kent State shootings on May 4, 1970, in which four students were killed by members of the Ohio National Guard. The song expresses outrage and grief over the killings and calls for an end to the Vietnam War. Specific lyrics referencing the event are:

"Tin soldiers and Nixon coming

We're finally on our own

This summer I hear the drumming

Four dead in Ohio

Gotta get down to it

Soldiers are cutting us down

Should have been done long ago"