Well, if the South was in Revolution, then it needed to justify that revolt with an appeal to an attack on its individual rights.
The Federal gov't was not abusing anyone's individual rights in the South.
As for the war being settled by force of arms, if the Colonists had lost the war that would not negated the rightness of their cause, just as if the South had won, it would not have justified their cause.
Fortunatly, the winners in both were those who were defending the just cause.
If Washington had not waged a brilliabnt campaign in New Jersey in December and January, 1776-77, the British would in all likelihood have defeated what they called the "Rebellion." Right cause of not, we would have remained British subjects.