Okay, I know it sounds ludicrous, but just hear me out.
Basically, imagine if the US responded to the protests regarding the Vietnam War like they did to ones in WW1... Full censorship, massive amounts of arrests, and other clamp downs like that. This causes many activist movements to go underground and radicalize, including the Civil Rights and Anti-War movement.
From this, in a few years, the pressure reaches the explosion point, and the country gets torn apart in a civil war, over issues of not just Vietnam, but also race like the first ACW, and perhaps class if the economy takes enough of a dip.
Now, why would the US government respond this way to protests? Well, they did so in WW1, so them doing so again is certainly not ASB. However, in what way they decide to respond will be... interesting.