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.
Article 1, Section 10, Clause 3
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 Founders understood that the federal government can threaten individual liberty, but so can the state governments. The Constitution recognizes threats from both actors and, therefore, contains specific limitations on both. Article 1, Section 9 limits the federal government; Article 1, Section 10 limits state governments.
Section 10 consists of absolute prohibitions on the states (e.g., prohibitions relating to military and monetary powers) and qualified prohibitions on the states (i.e., prohibitions that Congress may suspend).
Section 10, Clause 3 contains qualified prohibitions on a variety of activities. The prohibition on states charging duties of tonnage prevents state-specific protectionism and protects Congress’s commerce power. Because standing armies were a grave threat to the new republic, the constitution prohibits them at the state level. States may maintain militias, but not standing armies. But, the most significant portion of the clause concerns the ability of states to enter into agreements with foreign nations or other states. As Michael S. Greve notes in Compacts, Cartels and Congressional Consent, “For a federal republic, and especially for a nascent federal republic, the prospect of separate, unsupervised agreements among its member-states and between a member-state and a foreign nation must constitute a cause for alarm.”
The Articles of Confederation forbade the states from entering into an agreement with foreign powers. Additionally, any “treaty, confederation, or alliance whatever” among the states required congressional consent, and Congress would settle any disputes arising between the states. But the Articles of Confederation proved ineffective. The Constitution supplied a remedy. The Constitution created a new apparatus for the federal government to engage foreign nations: the president would be the chief actor in foreign affairs. He would negotiate treaties and, in turn, the Senate had to ratify treaties before they went into effect. Individual states could not enter into agreements or treaties with foreign nations. But, in the event of foreign invasion, an individual state could respond.
Agreements between the states pose threats to federal powers, to states not party to the agreement, and even to individual rights. By requiring such agreements to have the consent of Congress, other states would be informed of the agreement and able to protect their interests and the rights of their citizens. In many ways, congressional approval on state compacts was a compromise. James Madison wanted to give the federal government a much broader power over the state governments: specifically, he advocated a congressional negative on state laws. Delegates at the Convention compacts clause rejected Madison’s proposal—three times—as overly nationalist and unnecessarily broad. The Convention instead opted for federal supremacy over certain categories of activity, blanket prohibition of some activity, and congressional approval for any agreement between the states. Together these prohibitions mollified Madison’s concerns and protected against state governments’ encroachments on liberty.
Though the Compacts Clause makes clear that forming compacts is prohibited without the consent of Congress, it is not clear what form that consent must take. Does it require a law be passed and signed by the president? Or can Congress accomplish it without presentment? Nor does the clause specify whether Congress must consent prior to the formation of the compact. There is also debate about the scope of these compacts. Compacts prior to 1921 primarily concerned boundary disputes. Compacts in the later 20th century include complex regulatory schemes that may present separate constitutional problems. These ambiguities will likely be tested as states become more creative with the scope and substance of their agreements.
Julia Shaw is the Research Associate and Program Manager at the B. Kenneth Simon Center for American Studies, The Heritage Foundation.
 Michael S. Greve, Compacts, Cartels, and Congressional Consent, 68 M.L.Rev. 285, 296 (2003).