I'm a Southerner, but that's just plain factually incorrect. The seccession documents of EVERY single southern state name Federal intervention on slavery as the primary cause of their secession.
BOTH sides felt it would last a few weeks, months at the most. Remember the picnickers at 1st Bull Run? When it became apparent the South wasn't going to return on it's own, and the Northern armies wouldn't do what was necessary to end the rebellion, the war turned into one of attrition.
The secession of the Southern States was one of emotion rather than practicality.