Feels like a story half-told without looking past the war to the failure of reconstruction policy post-Lincoln.
I'm convinced that the failure of reconstruction explains the current state of US politics, perhaps better than an analysis of the civil war itself.
The author mentions that: "Southerners adopted beliefs that justified slavery as a moral good, such as the view that slavery benefited slaves because it offered them a better life than they would otherwise have as freed people."
The law was changed, but this belief was still held by the defeated southerner ruling class, who began using weasel tactics as a method of enforcement (example: Jim Crowe). These tactics were initially fought by federal power, but the political will to carry on this fight faded over time. I fret to think of the US that could have been, had this fight been given the attention it deserved.
This is why I feel like the people were told to wipe out every man, woman, and child in stories from biblical times. If you're going to do it, do it. Otherwise, people are left with sentiments that will fester and eventually boil over again. However, even in the biblical stories, that doesn't happen, and the two sides are bitter ever since.
Regardless, the only winning move (to any war) is not to play.
This does seem to very often be the pattern, which makes it pretty surprising to me in retrospect how smoothly Germany and Japan have adapted to the post-World-War-2 international order. They aren't completely nonexistent, but German Nazi groups and Japanese imperialist groups certainly have not dominated the post-war political scene, and democracy seems pretty firmly entrenched in both those countries.
So, perhaps you don't need to wipe out every man, woman, and child a la Deuteronomy, and there is some sort of post-war intervention that can be successful in creating a lasting peace. I don't have a much stronger conclusion than "sometimes it is possible", though.
Germany and Japan no longer romanticize and idolize revolutionary violence and nationalism, whereas the US very much still does in the mythology that its built around gun culture and the Founding Fathers - in no small part because the US suffered relatively little for its part in World War 2, and never actually learned the lessons the rest of the world did.
There's plenty of idolization of it in Germany still, with the prevalent myth of a "clean" Wermacht. Often the proponents of the myth end up unwittingly parroting transparent Nazi-era propaganda about the supposed heroic deeds of whatever random, obscure German military figure.
This is often overlooked by many political commentators who look at the otherwise unexplained, sudden popularity of Republicanism in the Sunbelt, and make up weird theories such as "the parties must have switched", or something. The reality is of course far subtler and more interesting than that!