Hmmm dunno. I don't know that much how things are now in the US. I follow some news, but being there or even any of the bordering countries would give a perspective i don't have.
I think their current divide started escalating around the Obama years. Before that i'd guess their closest point to a civil war was during Vietnam war. The cold war gave them something to unify against. The war on drugs and terror created more divides. And the last few years they've started to unify against each other. Way these things pop off usually takes time to brew and then a few small incidents set it off.