I will give you that. Even though the actions of the Founders predates the definition of treason as laid out in the Constitution their actions would certainly have qualified as treason by anyone’s definition.
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Secession is not treason. Sorry to disappoint you.
Secession as practiced by the Southern states was illegal. Sorry to disappoint you. And I'll agree that that in and of itself was not treason. Where the treason came in to play was when the Southern states chose to war against the government in order to further their aims.