Please, chapter and verse that proves that secession is illegal. In the US Constitution.
Second paragraph of Article VI: This Constitution, and the laws of the United States which shall be made in pursuance thereof; and all treaties made, or which shall be made, under the authority of the United States, shall be the supreme law of the land; and the judges in every state shall be bound thereby, anything in the Constitution or laws of any State to the contrary notwithstanding.
Third paragraph of Article VI: The Senators and Representatives before mentioned, and the members of the several state legislatures, and all executive and judicial officers, both of the United States and of the several states, shall be bound by oath or affirmation, to support this Constitution; ...
First sentence of Article III, Section 3: Treason against the United States, shall consist only in levying war against them, or in adhering to their enemies, giving them aid and comfort. The point of this is that it shows that the individual owes allegiance to the United States directly, and not through the state; otherwise there could be no such crime.
These provisions interact. It is not a defense to a charge of treason that one acted in accordance with state law, because the supremacy clause defeats any such law. It is, or should be, no defense, but a greatly aggravating circumstance, that one is an officer of government, federal or state, acting as such, since that is a breach of the oath of office.
I might also mention that the Articles of Confederation twice declare that the Union shall be perpetual. Granted, that word is not used in the Constitution; perhaps it was thought unnecessary in view of the provisions already quoted.