Before I get into the book, which is strongly recommended, it's worth reading a little about the author. That's what Wikipedia says. For our purposes, it will helpful to know the following as reported in the introduction:
One day, not long after Appomattox, he told his father he had reached the conclusion that slavery was wrong. The reply was, to the writer's surprise, that his mother in early life had been an avowed emancipationist; that she (who had lived until the writer was sixteen years old) had never felt at liberty to discuss slavery after the rise of the new abolitionists and the Nat Turner insurrection; and then followed the further information that when, in 1846, the family removed from South Carolina to Alabama, Greenville, Ala., was chosen for a home because it was thought that the danger from slave insurrections would be less there than in one of the richer " black counties."
What a creature of circumstances man is! The writer's belief about a great moral question, his home, his school-mates, and the companions of his youth, were all determined by a movement begun in Boston, Massachusetts, before he was born in the far South!
What better guide to the causes of the Civil War could a modern historian ask for?
We must now discuss Mr Herbert's thesis. Mr Herbert believes that the Civil War was unnecessary and tragic. He believes the war was caused by the abolitionist movement. By "abolitionist" he means people like William Lloyd Garrison and John Brown. He does not mean all people who opposed slavery (I will call this latter group "emancipationists" for the sake of simplicity). The difference between these two groups is that former believed in a higher law. Mr Herbert argues that everyone agreed that the Constitution allowed slavery in the states (perhaps "everyone" is too strong, but it is close enough to correct for the exceptions to be meaningless) but that the abolitionists believed in a higher law.
Consequently these abolitionists supported subverting the Constitution. They broke the Constitution – i.e. the compact between the states. Many of the abolitionists supported both secession (the North seceding from the South) and nullification. That is, they supported these doctrines prior to the South seceding, at which point they opposed them.
Mr Herbert's view is perhaps best summarized by a letter that he quotes in the book. The letter is written by George Ticknor, a northerner, and the quote is: "On the subject of our relations with the South and its slavery, we must—as I have always thought—do one of two things; either keep honestly the bargain of the Constitution as it shall be interpreted by the authorities—of which the Supreme Court of the United States is the chief and safest—or declare honestly that we can no longer in our conscience consent to keep it, and break it."
Unfortunately, the abolitionists followed a different path. They refused to keep the bargain of the Constitution as interpreted by the Supreme Court and the refused to honestly admit of the change in their position and peacefully allow secession. The Southerners were fighting for the original Constitution, while the Northerners had effectively refused to continue living under the original document.
So that Mr Herbert's position does not seem so radical, it's worth taking a minute to look at the actual consequences of the Civil War (as opposed to the consequences that we would have liked to see). The position of the blacks after the Civil War (inclusive of Reconstruction) was largely unchanged and arguably worse (at least from a materialistic standpoint). The ultimate outcome of the War was not freedom for blacks, it was domination of the South by the North and the destruction of the South.
Mr Herbert's work is really a reactionary masterpiece. It's ultimate, broad argument is that adherence to higher law above the written law is ultimately purely and solely destructive of all things good. The Civil War didn't end slavery (at least not in reality). It did, however, destroy a lot of life and lead to a new system of government for the states that was not chosen democratically, but was mandated by force and subjugation. Hell hath no fury like a Puritan following his higher law.