The vigor...and depth of hate toward fellow Americans....showed up on the brink of WWII....during WWI, certainly flared up when the
Maine cooked off in 1898...but the long built up hatred pre-dating the Civil War took 25+ years. Instead of seeing them as fellow Americans...they were seen as aliens,...internal enemies.
I am willing to identify the origin of this...and it caught the culture by surprise: 1960's Anti-war, Anti-government, Anti Patriotism radicals.
Nixon...and the bungling of our nation in the 1970's did nothing to reverse that. Look at the mockery that emerged among the Left during the Bicentennial. Reagan's earnest attempt to reawaken the love of country was met with a united media: You are a war-monger, you will get us in a nuclear war, you hate gays, your ideas are out of date, and your fundamentalist followers are closely related to Jim Jones.
That bias helped provide enough fuel that has created two ideological camps...with almost no common ground.
You see it every day right here. The Left has all its 5th columnists spread in the media, the universities & school districts, Progressive faux religionists, BLM & antifa activists, and now...compromised government agencies...Where can opposition coalesce? They move away from the establishment voices & lean toward confrontation....which both enrages /terrifies late night television hosts & the Young Turks.
So here we sit in angry opposition to each other...and no way to diffuse the ugly heritage of rivalry because each side has a distinctly different view of what it means to be American. One side is more prone to violence now...but I suspect the other is very eager to rise up.