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
Secession implies that states have the sovereign right to raise their own armies, negotiate treaties with other nations, etc. Since the things that go along with secession are illegal, by implication secession (at least secession by fiat) is as well.
Ok Adolf anything you say.