The view of the South was that the pact that they entered Union under was voluntary. Since they felt the terms of the contract had been violated, they surmised that they had a legal right to separate themselves from the Union.
The Union took the position that the agreement of the original states was forever binding. However, during and after the war the North treated the South in two different ways: in some occasions, the South was implied to be wayward souls, as secession was not possible. After the war, the North set up requirements for the South to return to the Union, which of course implies that they left and secession did occur.
Why did the North have these two opinions? How did this affect the way the South was treated before, during and after the war?