The south basically tried to enact a right to "Self-Determination", yet under those same International laws they would have to allow any other territories in their respective area to peacefully dissolve into their own sovereign areas, which many planned on. If the south would have won the war it would have been splintered even more, even to the point of sovereign city-states. That would have wrecked havoc on the economies of everyone leading many to petition for the right to join the Federal Union. Another issue with the south's aggression is that America had to protect the citizen's rights of the south to "National Identity", which was, and is, an internationally recogonized right of any citizen to be a nationalized citizen of a parent country...not all of the people in the south believed in rebellion.
no, the south's duly elected representatives attempted to withdraw from the union, due to obvious, and deliberate abuses by the north. the northern states had the southern states's balls in a vise and they just kept crankin that handle.
the north (for all the blandishments and modern revisionism about slavery) did not want to lose their control over the south which was rapidly becoming a servant of the moneyed interests in the north. the south was being used as an ATM machine by the northern establishment, while making transparent and obvious moves to ensure the south stayed on the hind tit (which it has been on since the Late Unpleasantness)
the south would not have dissolved into city-states any more than the philippines or panama did.
the secessionist movement had nothing to do with "international law" in fact the concept of "international law" did not exist save the law of the sea. the north didnt protect shit, certainly not "national identity", and "national identity" is just another modern concept promulgated by progressives and political science majors, not a classical european idealists. the antebellum south was not a bastion postmodernist thought or political science navel gazing. i dont know who is telling you this garbage but i bet they have a greying ponytail and birkenstocks.
when Nathan Hale said "I regret that I have but one life to give for my country" he was talking about Connecticut, not the Continental Congress. the union of states was intended to be a UNION of STATES not a fedral nationstate with subservient provinces. the founders well well aware of the nationstate as there were several at the time, and they were also well aware of the nationstate with subservient provinces, as the feudal system depends on this arrangement. they deliberately chose to NOT have a dominant federal government, but the ensure the federal government is a federation that serves the states which is why we call it a federal system today despite it's current status as a dominion rather than a federation.
we were supposed to be a republic formed from many independent republics, with the federal government mediating and regulating the actions between the United States (again thats why we call it that) and foreign powers,, and ensuring the freedom of commerce between the member states through MUTUAL CO-OPERATION not commands and fiats from washington dc. a single "national identity" was never part of the plan, nor was compulsory membership when the union does not serve the ionterests of the states any more. this is why the declaration of independence, the federalist papers, and the constitution make clear that dissolution of governments is the RIGHT of the people when the consensus of the voters is that the current form is not working, even by violence if need be.
read more founders and less post modern revisionist history.