This is a constitutional detective story.

Several months ago, I was faced with this question: Did the Founders’ Constitution give Congress the power to restrict immigration? Or was this a subject reserved to the states?

The question had come to the fore because of the controversy surrounding the Arizona immigration law. My own search for the answer offers some important lessons about constitutional interpretation.

The Constitution, as readers of this website know, grants Congress only certain enumerated federal powers. About half of these are found in Article I, Section 8, while the rest are scattered throughout other parts of the document. Yet none of those powers explicitly mentions immigration.

This apparent silence has led some to suggest that immigration was left exclusively to state control. However, the Founders gave primary control over foreign affairs to the federal government, and immigration (and emigration) was an important aspect of foreign affairs in the eighteenth century. Also, Article I, Section 9, Clause 1, which barred Congress from prohibiting before 1808 the “Migration” of free people as well as “Importation” of slaves presupposed a congressional power to prohibit or restrict immigration after 1808.

But if Congress has power to regulate immigration, where in the Constitution was it granted?

Some writers have argued that it was part of Congress’s authority to “regulate Commerce with foreign Nations.” This sounded plausible, but as I thought about it more, I became troubled. From reviewing hundreds of eighteenth-century sources, I had learned that “commerce” nearly always referred to the activities of merchants and certain closely-related activities. These activities certainly encompassed travel for business purposes and travel by ship or other conveyance. But constitutional scholar David Kopel pointed out to me that those activities did not include the fellow who hoofed it over the international border to live in the United States. An immigrant of that description was not engaged in “commerce,” as the Constitution uses the word.

So I began another search to learn whether there was a federal power over immigration, and if so where it came from. Eighteenth-century law provided the answer – not commercial law, but international law.

Article I, Section 8, Clause 10 of the Constitution granted power to Congress to “define and punish . . . Offences against the Law of Nations.” I decided to dig more deeply into the eighteenth century legal sources to determine whether that might include authority over immigration. Sure enough, it turns out that during the Founding Era, restrictions over immigration and emigration comprised a well-recognized branch of the “Law of Nations.” In other words, Congress’s power to “define and punish . . . Offenses against the Law of Nations” included authority to “define” immigration rules and “punish” those who violated them. An explanation appears in latest update of my book, The Original Constitution: What It Really Said and Meant .

Why is this constitutional detective story significant? First, clarifies why the constitutional text assumes that after 1808 Congress could regulate “Migration” from foreign lands. Second, it clarifies that Congress cannot use the interstate commerce power to bar non-commercial travel within the United States. Third, it knocks one of the props out from under an argument that, however silly, is solemnly advanced by some “liberal” writers – that “commerce” included non-business travel, and therefore that “commerce” also included nearly all other human relationships.

Finally, this story underscores a point I explain for the layperson in The Original Constitution: When the Constitution is unclear, eighteenth century law offers us valuable trail marks toward the truth.