As is anything else the government deems - appeal to force.
unconstitutional,
Not obvious, outside the notion that a militarily victorious government so stipulated. Any other decision would have been death for the republic as it remained.
impossible,
Appeal to force. The willpower of the secessionists was vastly underestimated by Lincoln and it cost him dearly - are you so quick to follow his mistake?
and meaningless.
Apparently not meaningless to Lincoln and his Northern followers who staged an invasion to supress it.