The State has to abide by that same contractual obligation for Legal purposes according to Constitutional Law. It is why right-wingers are nothing but immoral hypocrites about illegality in border threads.
No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility.