In a nation premised on consent of the governed, Informing a state that it's destiny may be better served in another coalition is a perfectly reasonable thing to do.
And that, good sir, is plain old fashioned treason.
No it isn't. Under the understanding of that era, the states were free and could do what they wanted. Even Chief Justice Salmon P. Chase said that "Secession isn't Treason."
It was the way the slavocracy practiced it.