I do not think you have read a great deal about the Civil War. The author is absolutely correct that friendships continued during the war. Officers on both sides were often saddened to hear of the death of a former comrade who was on the other side.
I have ancestors who were brothers and on opposite sides.
There were a few stories of this, particulary near the Mason Dixon line where soldiers truly were neighbors with opposing forces.
But another point to consider is that during the Civil War, neither side _wanted_ a war. It was truly the last thing anyone was looking for, and the last thing anyone wanted to participate in. Regardless of what of our movies showed us.
There were families to raise, farms to tend and each soldier left behind people who needed them to be alive and effective.
In our next war, we will see two factions who WANT war, will have the ability to care for those left behind (Through tech and supply chain improvements) and will have a lesser problem going to battle with the next town over.
Newark NJ would go to war with Bayonne NJ today, much less an orange county New Yorker supporting the carpet bombing of a west virginia coal town.