To determine that, we will need two passages; here is the first:
However illegal and unbearable the oppression, however lofty and noble the motives of the rebels, and however beneficial the consequences of their violent resistance,
(1) a revolution is always an illegal act, disintegrating the established order of state and government. …
A revolution is an act of warfare between the citizens, it abolishes the very foundations of legality and
(2) is at best restrained by the questionable international customs concerning belligerency.
If victorious, it can afterwards establish a new legal order and a new government. But
(3) it can never enact a legal “right to resist oppression.”
Such an impunity granted to people venturing armed resistance to the armed forces of the government is tantamount to anarchy and incompatible with any mode of government. (HA, 286n)
Here is the second:
The right of self-determination in regard to the question of membership in a state thus means:
whenever the inhabitants of a particular territory, whether it be a single village, a whole district, or a series of adjacent districts, make it known,
(4) by a freely conducted plebiscite, that they no longer wish to remain united to the state to which they belong at the time, but wish either to form an independent state or to attach themselves to some other state,
(5) their wishes are to be respected and complied with. This is the only feasible and effective way of preventing revolutions and civil and international wars.
…the right of self-determination of which we speak is not the right of self-determination of nations, but rather the right of self-determination of the inhabitants of
(6) every territory large enough to form an independent administrative unit.
If it were in any way possible to grant this right of self-determination to every individual person, it would have to be done. This is impracticable only because of compelling technical considerations… (Liberalism, 109ff)
It follows that if the United States before the War between the States was a voluntary union of the states, and if each state had rights against federal encroachments, and if in addition those rights included a legal right to secede, then
according to (1) the federal government aggressed illegally on the seceding territories.
The feds had no right to assist the blacks in “resisting oppression,” or so Mises argues in (3), by overthrowing the legitimate government.
Further, slavery or not, the Southern states had a moral right to secede, according to (5), and
if moreover secession was conducted in harmony with (4) (which it was), and
the Confederacy was sufficiently large and was in fact for all intents and purposes a distinctive nation and culture as per (6) (which again it was), and finally
if the rules mentioned in (2) were not followed (which they weren’t), then:
those states were in the right, and the Northern states were initiators of a violent and barbaric revolt against which the Southerners had every right to defend themselves.