The Compact Clause in the U.S. Constitution requires approval by Congress of any agreement among states to increase their power at the expense of federal power. A comparable prohibition had been in the Articles of Confederation, but the Framers chose even stronger language for the U.S. Constitution:
"No State shall, without the Consent of Congress ... enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such iminent Danger as will not admit of delay." (Article I, Section 10, Clause 3)
The U.S. Supreme Court has interpreted this clause narrowly by allowing state compacts as long as they do not "encroach on the supremacy of the United States," which is protected anyway by the Supremacy Clause. United States Steel Corp. v. Multistate Tax Commission (1978).