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> weakness of the federal government. ... meant there was no monopoly on military power to enforce abolition on resistant states.

still digesting the rest of your reply, but on this point: prior to the civil war, the US army had made its units up out of people from the local area. In the run up to the civil war, some of these units defected en masse, with their command structure intact. Ever since, the army has been careful to rotate geographically diverse people through each unit, ensuring spatial and temporal mixing.

(if chatter surrounding events of the past few years {Bundy, Floyd, J6} is to be believed, the army is not worried about the ability of its officer corps to distinguish between legal and illegal orders as it is the inclinations of potential splinters at the squad level)