As I said, there some that would consider it treason, some do not. Some consider secession legal, some do not.
“The north had imposed it’s opinion of blacks in the South by having them count as 3/5’s of a person for representational purposes.”
The South’s position was that slaves would count one for one for the purposes of representation in Congress. The Northern position was that slaves would not count at all for the purposes of representation. The solution was not “imposed” it was a compromise, agreed to by all that slaves would count for only 3/5ths of a person for representative purposes.
“The South left, peacefully”. Then proceeded to steal several million dollars of United States Property, fire on ships flying the flag of the United States, fire on a United States Amy installation and issue letters of Marque allowing southern sea captains to capture American flagged ships. All of this done without the United States having fired a shot.
The north proved that secession was legal.