In 1783, Great Britain made peace with 13 separate nations, the 13 American Colonies. They went on to union and formed one country, the United States of America in a Confederation of States, where all of the states were bound together in a Congress, they formed a Federal gov. with representatives from all states in Congress. The Constitution said nothing about secession of a state, and the president prior to Lincoln, Buchanan, stated he had no right to send troops to force South Carolina back into the union of States. Since this is the case, why was reforming all the seceded states back into the union such a big deal for Lincoln?
The word Constitution itself, unlike the words "confederation; compact; treaty; league" is a unilateral agreement. Akhil Reeed Amar (legal historian) argues that the Constitution, unlike the Articles of Confederation, specifically created an irreversible union between the States.
Geographically, the most important port of the US was and still is the city of New Orleans in Louisiana, mainly thanks the the Mississippi river and its estuaries across North America all the way up to the modern-day Rust Belt, at that point the industrial heartland of the US.