How sad that even Dr. Rahe is unable to recognize the emptiness of his conclusion. Abraham Lincoln worked hard to justify a "repudiation of the principles enshrined in the Declaration of Independence." Did that indicate it was necessary for the Union?
I will say then that I am not, nor ever have been in favor of bringing about in anyway the social and political equality of the white and black races - that I am not nor ever have been in favor of making voters or jurors of negroes, nor of qualifying them to hold office, nor to intermarry with white people; and I will say in addition to this that there is a physical difference between the white and black races which I believe will forever forbid the two races living together on terms of social and political equality. And inasmuch as they cannot so live, while they do remain together there must be the position of superior and inferior, and I as much as any other man am in favor of having the superior position assigned to the white race.Plus, he misses how economic and legal factors don't rely upon the justification. We often find legal decisions that are not just to a third party, for example.
I say upon this occasion I do not perceive that because the white man is to have the superior position the negro should be denied everything. I do not understand that because I do not want a negro woman for a slave I must necessarily want her for a wife. [Cheers and laughter.]
[...]
I give [Douglas] the most solemn pledge that I will to the very last stand by the law of this State, which forbids the marrying of white people with negroes. [Continued laughter and applause.]
I will add one further word, which is this, that I do not understand there is any place where an alteration of the social and political relations of the negro and the white man can be made except in the State Legislature---not in the Congress of the United States---and as I do not really apprehend the approach of any such thing myself, and as Judge Douglas seems to be in constant horror that some such danger is rapidly approaching [...]--Abraham Lincoln
The root causes are much more diverse, but I believe that nearly every cause can somehow be linked back to slavery and land -- the South's dependence on expansion of those two resources, and their realization of the trap they had set for themselves with the Compromise of 1850, put them in desperate realization that they were ultimately facing checkmate.
Once the North had gained the majority in the Senate, and with the ensuing melee over Kansas statehood, the South realized that expanding slavery into new territories would not happen. At that point, the only thing that would have prevented the Civil War would have been the tractor.