...the right to secession could be determined in trial by battle. It was a very popular opinion at the time that the legality of secession had been decided on the battlefield.

She writes that Americans "faced the uncomfortable realization that their society, which prided itself on enlightened rationalism and adherence to the rule of law, had revived a repudiated medieval superstition to confront secession. They were forced to ask themselves whether they had become barbarians in the upheaval of the war. For many introspective Americans the answer was an uncomfortable 'yes.'"

The question of whether the legal issue had been/could be resolved by war was hotly debated at the time. Ultimately there was a decision to rely instead on the rule of law, which is why a trial was necessary.

I'll have to buy the book to get the chapter titled "The Return of the Rule of Law" so I'm not sure how she gets there.