McPherson: The South professed to be fighting for self-government. The thirteen colonies had seceded from the British Empire based on a philosophy of the freedom of people to choose their own form of government The Southern leaders in 1861 said they were fighting for the same rights. That was their definition of liberty.
There was a corollary to that definition. An essential component of liberty is the protection of private property. Slaves, of course, were property. To deny Southerners the right to take their slaves into new territories acquired by the United States would be a violation of their rights of property, therefore of their liberty. So they could, with perfect sincerity, claim that they were fighting for liberty even though part of that liberty was their right to hold slaves and to take them into any part of the territories acquired by the United States in the same way they could take personal property or livestock or anything else.
The Northern definition of liberty was the preservation of the Union, the nation, based on that revolution of 1776. They feared that recognizing the right of secession would undermine the whole concept of a government based on majority rule, constitutional procedures, and democratic elections. So they were fighting for their concept of liberty
Text of full interview:
http://www.neh.gov/whoweare/mcpherson/interview.html
Terry