In addition to granting the government powers to regulate trade and raise revenue that it either lacked or could not enforce under the Articles of Confederation, the Framers intended the Constitution to centralize much, if not all, power over foreign affairs. Many of the federal government’s enumerated powers relate to foreign affairs and have corresponding restrictions on states in Article I, Section 10. Article VI of the Articles of Confederation had permitted the states to conclude treaties with foreign governments with the consent of Congress. States could also grant letters of marque and reprisal after Congress had declared war. While some of Article I, Section 10’s proscriptions, such as the ability to levy tonnage duties or enter into “compacts or agreements,” may be permitted by Congress, others, such as the prohibitions described here, are absolute.

Treaties, as well as alliances and confederations, are formal, binding agreements between nations that are the subjects of international law. “Compacts and agreements” are usually made by governmental officials, such as the executive, or by subsidiary governmental units, such as states or municipalities. In the late eighteenth century, governments issued letters of marque and reprisal to authorize private ships to attack certain foreign shipping and gain booty for their efforts. Issuing them was regarded as an act of war.

In The Federalist No. 44, James Madison noted that these proscriptions (like the prohibition on treaties) either “need[ed] no explanation” or (like the restrictions on letters of marque and reprisal) were “fully justified by the advantage of uniformity in all points which relate to foreign powers; and of immediate responsibility to the nation in all those for whose conduct the nation itself is to be responsible.” Justice Joseph Story concurred, writing in his Commentaries on the Constitution of the United States (1833) that the power to issue letters of marque and reprisal “is appropriately confined to the national government” because “the protection of the whole Union is confided to the national arm, and the national power,” and no state “should possess military means to overawe the Union, or to endanger the general safety.” As noted foreign-affairs scholar Louis Henkin remarked, “these restrictions are as clear as words can make them and have raised no issues. . . . ”

The courts have had little occasion to deal with the clause, though in Holmes v. Jennison (1840), Justice Roger B. Taney, writing for himself and three other Justices, commented that the clause “positively and unconditionally” forbade states from entering into treaties, and that “even the consent of Congress could not authorize” them to do so. Taney, citing Emmerich de Vattel (1714–1767), also distinguished formal “treaties,” which were expressly forbidden to states, from “agreements” and “compacts,” which Congress could authorize.

The Supreme Court has not had an opportunity recently to distinguish between a “treaty” and a foreign “compact” or “agreement.” In fact, in U.S. Steel Corp. v. Multistate Tax Commission (1978), the Supreme Court described the original understanding of the terms as historically “lost.” It may be that Court would consider the distinction a political question. As one author has noted, states and municipalities have recently entered into a number of such compacts and agreements without the consent of Congress and have attempted to distinguish these compacts and agreements from treaties by inserting language that these agreements constitute political commitments with no legal effect. Another reason that many state-initiated compacts and agreements have not been challenged in court as violating the State Treaties Clause is that the executive branch has encouraged such agreements. Michael Ramsey argues that the founders did indeed distinguish between treaties and compacts, drawing upon ancient Roman distinctions maintained by authorities such as Hugo Grotius (1583–1645) and de Vattel. Ramsey also asserts that states reserve all foreign affairs powers not delegated to the federal government or prohibited to the states by the Constitution.