What did the riots at the 1968 Democratic National Convention and the Kent State protest in 1970 have in common?

Anti-war protests and violent police reactions.

Both of the riots began as youth protests against the Vietnam War as well as civil rights concerns and concerns over the direction of the government.

In both cases, police forces were mobilized to control the protesters but the forces used aggressive and violent means. In Chicago, police released tear gas and used violence against the protesters. At Kent State, the National Guard was sent to control the protest but ended up firing on the crowed of protesters. Both of these protests turned into riots when police forces used violence toward the protesters.

Both protests were against war. Anti-war protests were carried out to disrupt meetings and prevent war in Vietnam and the US.