The author is looking at the war from one side only, he needs to look at the southern motivation. From the Northern point of view it wasn't about slavery, but from the southern one it was.
"Texas abandoned her separate national existence and consented to become one of the Confederated Union to promote her welfare, insure domestic tranquility and secure more substantially the blessings of peace and liberty to her people. She was received into the confederacy with her own constitution, under the guarantee of the federal constitution and the compact of annexation, that she should enjoy these blessings. She was received as a commonwealth holding, maintaining and protecting the institution known as negro slavery-- the servitude of the African to the white race within her limits-- a relation that had existed from the first settlement of her wilderness by the white race, and which her people intended should exist in all future time. -- Texas Declaration of the causes of secession
Wrong answer Gustav! From the Southern point of view it was about a State's Right of FREEDOM of Self Determination. Slavery was merely a means to economic prosperity for the Plantation owner, however the Southerners were also trying to maintain an economic power parity with the greedy merchants in the North. The damnYankee inhabitants of the Northern States dispised the Southerners culture, thereby they were using slavery as an object to incite anger and hatred against the Southern people. The abolitionists were as zealous as a lot of Muslim extremists are today.
Under the Founders' you must remember that the State Governments had more administrative cognizance of their citizens, and Washington DC had only limited influence over a State's internal affairs. I think this slogan from a Confederate flag summed up the South's position best -
"WE CHOOSE OUR OWN INSTITUTIONS"