U.S. Constitution - Section 9, Clause 3: 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 delay.That is a restriction on the several states, not on the federal government.