Article I, Section 10, 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 of delay.
The Constitution makes no provision with regard to the time when the consent of Congress shall be given or the mode or form by which it shall be signified. 1 While the consent will usually precede the compact or agreement, it may be given subsequently where the agreement relates to a matter which could not be well considered until its nature is fully developed. 2 The required consent is not necessarily an expressed consent; it may be inferred from circumstances. 3 It is sufficiently indicated, when not necessary to be made in advance, by the approval of proceedings taken under it. 4 The consent of Congress may be granted conditionally
upon terms appropriate to the subject and transgressing no constitutional limitations. 5 Congress does not, by giving its consent to a compact, relinquish or restrict its own powers, as for example, its power to regulate interstate commerce. 6