The supremacy clause.
You're going to have to accept the idea that the Supreme Court in 1863 referred to the secessionists as traitors.
"By the Constitution, Congress alone has the power to declare a national or foreign war. It cannot declare was against a State, or any number of States, by virtue of any clause in the Constitution. The Constitution confers on the President the whole Executive power. He is bound to take care that the laws be faithfully executed. He is Commander-in-chief of the Army and Navy of the United States, and of the militia of the several States when called into the actual service of the United States. He has no power to initiate or declare a war either against a foreign nation or a domestic State.
But by the Acts of Congress of February 28th, 1795, and 3d of March, 1807, he is authorized to called out the militia and use the military and naval forces of the United States in case of invasion by foreign nations, and to suppress insurrection against the government of a State or of the United States.
All persons residing within this territory whose property may be used toincrease the revenues of the hostile power are, in this contest, liable to be treated as enemies, though not foreigners. They have cast off their allegiance and made war on their Government, and are none the less enemies because they are traitors."
-- Prize Cases majority opinion.
Walt