Just a note. The Founders considered the several States to be Sovereign independently. That's why the Declaration of Independence refers to "Free and Independent States" (plural, not singular.) A "State" is not by definition a sub-jurisdiction of a larger political entity. A State is a country, a nation ("Nation-State" or "City-State" to use historical examples). Legally, the United States is a confederation of nations or states. Therefore, the Southern States are independently sovereign, the same as the Northern States. The question of the Civil War centers on one issue: Is secession legal? If it is, then the war prosecuted by Lincoln would be illegal under the Constitution. And if the States are sovereign, then secession is in fact legal.