One country, any country, can not exercise imminent domain in another country Under international law the Union would have had a legal right to supply Ft Sumter by sea just as the Russians, Ukrainians, etc, have a legal right to transit the Bosporus to gain access to the open sea. Just as the US has a right to transit the Gulf of St. Lawrence (Canadian) to gain access to the Great Lakes.
Once a nation cedes sovereignty over a piece of territory it can not unilaterally negate it. No Welshing. No Indian giving. We paid Spain for the Florida territory, France for the Louisiana Purchase, Mexico for the New Southwestern teritories (CA, AZ, NM, NV, CO, etc), Mexico Again for the Gadsden Purchase, Spain agains for Cuba, Puerto Rico, Guam, Philippines, and finally Denmark for the Virgin Islands. Now who here would claim that those countries have a right to unilaterally change their minds and reclaim those territories.
American independence was not unilateral. The colonists declared their independence but that did not make then independent. They became independent in 1784 by the ratification of the Treaty of Paris whereby Great Britain gave independence to the original 13 colonies. GB gave independence to each colony separately, not to the "United States". It is this fact that justifies southern secession as these 13 new nations which became independent under the Treaty of Paris never subsequently transferred or yielded that independence to any country.
Unfortunately the South has never been the intellectual heart of America and the same stupid, stiff necked, hot headed mentality that lead the South to its tragic end is still being perpetuated in certain segments of southern culture. Strange isn't it how some people revel in reliving the the stupidity of their forbearers and glorifying their own ass kicking. Surely there is a masochistic streak in the south as wide as the Bible belt.