Complex legal, Constitutional convolutions with Congress are not required to strip one of ones citizenship. If a State forcibly seceded from the Union, the residents of said State would be citizens of the new nation, say Texas for example. This is the gist of the thread, because one of the main objections herein voiced in opposition to secession is the change in citizenship forced on the residents of the State who did not support the secession.
I’ll remind the reluctant residents hereabout that when the Colonists decided to fight England for independence in 1775, not all of the colonials were in favor of rebellion. According to my research, about 15 to20% of the colonials were actively in opposition to the rebellion.
Having some substantial number of residents oppose secession is not in itself sufficient cause to discount the idea of secessionism.