Government by consent.
So if a state has a referendum and votes to secede then is that government by consent?
I don't blame Lincoln for putting down the rebellion once the Confederate states seceded, nor do I buy the argument that putting down secession is in any way illegal or unconstitutional. I blame Lincoln (and even more so the radical abolitionists of the time)for allowing relations between northern and southern states to fester to the point that the southern states were backed into a corner and forced (in their opinion) into what was ultimately illegal acts of secession. Most of the disagreements between north and south could have been negotiated by a more adept and competent and less ideological government at the time.
Section 10 - Powers prohibited of States
No State shall enter into any Treaty, Alliance, or CONFEDERATION; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility.
No State shall, without the Consent of the Congress, lay any Imposts or Duties on Imports or Exports, except what may be absolutely necessary for executing it's inspection Laws: and the net Produce of all Duties and Imposts, laid by any State on Imports or Exports, shall be for the Use of the Treasury of the United States; and all such Laws shall be subject to the Revision and Controul of the Congress.
No State shall, without the Consent of Congress, lay any duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.