Hi there!

Not to preclude further discussion, by /u/freedmenspatrol has previously written this excellent post on the legal basis of the South to secede and the basis for the Union to intercede and prevent that secession.

For a comprehensive overview of the causes of the Civil War - both the South's motivation of slavery and the North's motivation of saving the Union, please see /u/Georgy_K_Zhukov's piece here.