The US has a developing extremist problem, period. Having two sides with radicals willing to continuously ramp up the levels of violence, outrage, and intimidation is a recipe for bad things to happen. Another example, more recent than the period between world wars:

https://en.wikipedia.org/wiki/Years_of_Lead_(Italy)

In The Power of Nightmares, Adam Curtis talks about how opposing sides exploit the mutual vicious cycle of outrage. The common interest between extreme groups who otherwise demonize and hate each other, is that they both benefit from the cycle of outrage and the breakdown of society. Both fringes want to let things break down and roll the dice, for the chance they might come out on top.