Hi Micro:
(1) States joined voluntarily - with no clause forbidding them from leaving.
(2) The war was NOT fought over slavery. That is a wives tale. Abraham Lincoln made it explicit that he would have been willing to let slavery remain; he simply didn't want to allow secession. Lincoln felt it should be kind of like signing up to be a Crip.
(3) It WAS about perceived lack of representation. I believe that if you aren't represented you should be allowed to leave.
(4) The US was FOUNDED upon "No taxation without representation." So, if the South wasn't permitted to leave, the US had no moral right to seek freedom from Britain.
All that said, I also think the Blacks should have been freed -- and free to leave, as well, as they weren't represented. Also, the Civil War passes my economic standard for a war "won", given the North got to keep all the South's resources until this time.
I also want to point out that the majority of Southerners did not own slaves and did not want slavery. They supported their states breaking away because -- as I asserted above -- they did not feel they had national representation; the Southern economy was just too different from the industrial Northern economy. And "no taxation without representation" was the primary PREMISE of the US.