Section 8- Powers of Congress
To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions;
To provide for organizing, arming, and disciplining the Militia, and for governing such Part of them as may be employed in the Service of the United States, reserving to the States respectively, the Appointment of the Officers, and the Authority of training the Militia according to the discipline prescribed by Congress;
To "suppress Insurrections" are those Insurrections within a territory or state, not to suppress a State that rejects Federal law. A request from a State for U.S. Congressional military aid would have to be put forth before Congress could act against an insurrection. This clause was added to protect the slave states from internal slave insurrections.
The State is soveriegn and has a right to self defense and free from fear of being attacked by other states. The Union of American Colonies was formed for their protection in times of war, and again in during the American War for Independence. That Union was based on mutual respect between states of their soveriegnty and right to self determination.
The Supreme Courts 1869 decision is moot in comparision of the laws of 1860-61; it is also based on a dead law, the Articles of Confederation. The legal precedent for secession exist in the formation and ratification process of the U.S. Constitution. Whereas in the 1780s there existed two governments in the United States, the Confederation and the United States. Each state was free to ratify or not ratify the Constitution with the belief they could leave that Union under the statments of ratification of several states.
"WE the Delegates of the people of Virginia, duly elected in pursuance of a recommendation from the General Assembly, and now met in Convention, having fully and freely investigated and discussed the proceedings of the Federal Convention, and being prepared as well as the most mature deliberation hath enabled us, to decide thereon, DO in the name and in behalf of the people of Virginia, declare and make known that the powers granted under the Constitution, being derived from the people of the United States may be resumed by them whensoever the same shall be perverted to their injury or oppression, and that every power not granted thereby remains with them and at their will..."