To give just a little flesh to dot.hawkes correct but somewhat enigmatic answer, a brief review of some historical documents is instructive...
On December 20, 1860, South Carolina became the first state to secede from the Union. In its secession document, South Carolina boldly proclaimed to the world why it left the Union. Note its repeated emphasis on preserving slavery:
[A]n increasing hostility on the part of the non-slaveholding states to the institution of slavery has led to a disregard of their obligations. . . . [T]hey have denounced as sinful the institution of slavery. . . . They have encouraged and assisted thousands of our slaves to leave their homes [through the Underground Railroad]; and those who remain have been incited by emissaries, books, and pictures to servile insurrection. . . .
This was followed in short order on January 9, 1861, by Mississippi , when it became the second state to secede. In its secession document, it set forth the reasons it left the Union:
Our position is thoroughly identified with the institution of slavery - the greatest material interest of the world. . . . [A] blow at slavery is a blow at commerce and civilization. That blow has been long aimed at the institution and was at the point of reaching its consummation. There was no choice left us but submission to the mandates of abolition, or a dissolution of the Union, whose principles had been subverted to work out our ruin.
Each succeeding state also enunciated that peculiar institution as being primary in its decision to secede. (Source: Edward McPherson, The Political History of the United States of America During the Great Rebellion).
Cont.