The Constitution - specifically where it states that the Constitution is the supreme law of the land.
The "Supreme Law of the Land" stated:
The "Supreme Law of the Land" thus stated that, in view of the silence of the Constitution regarding secession and the absence of a Federal law regarding secession, then secession was a "power not delegated to the United States by the Constitution, nor prohibited by it to the States" and was therefore "reserved to the States respectively, or to the people."
What is secession, other than the declaration by a state that the Constitution is not the supreme law of the land?
In view of the clear wording of the Tenth Amendment, secession was a power "reserved to the States respectively, or to the people."
If you exercise an "out option" in a contract, you are not violating the contract. You are merely exercising your contractual rights.
After exercising that contractual option, said contract then no longer applies you but remains in force for those parties that have not yet exercised their "out option".
To simply ignore the plain language of the Tenth Amendment, now THAT'S violating the "Supreme Law of the Land".
Secession had been bounced around in the U.S. since the Hartford Convention. If it was to be outlawed, then the "supreme Law of the Land" required that a Constitutional Amendment be passed to that effect.
In the absence of such a Constitutional Amendment, the clear language of the Tenth Amendment remained in force.