Article I Section 10

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. 

No State shall, without the Consent of the Congress, lay any Imposts or
Duties on Imports or Exports, except what may be absolutely necessary for
executing it's inspection Laws: and the net Produce of all Duties and
Imposts, laid by any State on Imports or Exports, shall be for the Use of the
Treasury of the United States; and all such Laws shall be subject to the
Revision and Controul of the Congress. 

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.