A lot of what governs a nation isn't in its constitution but is a part of tradition. For instance, say Marbury vs. Madison hadn't ever happened and the supreme courts role as constitutional arbitrator were never established? It's not mentioned in the constitution. At best it's one possibility that could flow from the penumbras of the constitution.
The civil war definitely did set the tradition on the side of the nationalists and strong federalists, and it was established in much of law that nullification and succession were illegal acts - although, really, it's only by tradition that the supreme court won its shaky position as arbitrator and succession and nullification lost their shaky positions. The weak federalists and confederalists could theoretically reverse that tradition in the future by something equally as dramatic.
So you are saying the following statement is illegal?
We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness. That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, That
whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness. Prudence, indeed, will dictate that Governments long established should not be changed for light and transient causes; and accordingly all experience hath shewn, that mankind are more disposed to suffer, while evils are sufferable, than to right themselves by abolishing the forms to which they are accustomed. But when a long train of abuses and usurpations, pursuing invariably the same Object evinces a design to reduce them under absolute Despotism,
it is their right, it is their duty, to throw off such Government, and to provide new Guards for their future security.
-----------------------------------------------------------
Waterhead, that was the founding principle which this nation was established upon. How can it be "illegal" without our entire premise for existence as a nation being illegal? When the government becomes destructive, it is not only our right but our DUTY to overthrow it.