"When the Constitution says 'We the People' that's not just a rhetorical flourish. That's a description of the nature of the Union. See Akhil Reed Amar's excellent book The American Constitution: A Biography for this interpretation in full. Why did they do this? Because the founders did not want the national government to be a creature of the states. They had had one of those in the Articles of Confederation, and it didn’t work for them. That’s why the Constitution is very clear in Article VI that it supersedes the states; that's why all federal and state officials MUST swear or affirm their allegiance to the U.S. Constitution, look it up. The nature of the American Union, then, is based on popular sovereignty, the idea that the people have the right to rule. The American people spoke during ratification and created a new federal government in which they vested their sovereignty. The federal government is not merely an agent of the states, as John C. Calhoun asserted; it was not and is not a compact between states. The founders specifically avoided that. So, if a state wants to leave the Union, the only possible way is for 'We the People' to agree to let it go. But there is no specific mechanism for secession in the constitution as it stands. And really, there is no way to read a right of secession into its text. It isn't there, and that's because the Founders never intended for states to break away. Therefore, secession, which would effectively destroy the Constitution, was and is illegal. And Lincoln was simply carrying out his oath of office to 'preserve, protect, and defend' it."