The Constitution does indeed permit immigration caps as part of ‘the law of nations’
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Some pro-immigration activists question whether the federal government has any constitutional power over immigration. “Where,” they ask, “is the word ‘immigration’ among the powers the Constitution grants to Congress?”

This question has embarrassed many who favor restrictions. They have cast around for answers, but by and large, their answers have been unpersuasive. For example, they have argued the Constitution granted implied power to restrict immigration because such power is core to sovereignty. They also claim immigration is within Congress’s authority to govern naturalization and regulate commerce.

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There are problems with these answers. As shown by the wording of the Tenth Amendment (reserving unlisted powers to the states and people), the founders rejected the idea of implied sovereign powers. They did not see immigration law as merely a part of naturalization law. And they did not think it was “commerce” when a nonmerchant simply walked or rode a horse across an international border.

 

Immigration control advocates are on firmer ground when they highlight the part of the Constitution that reads, “The Migration or Importation of such Persons as any of the States now existing shall think proper to admit, shall not be prohibited by the Congress prior to [1808].”

This provision covered both slaves and free immigrants.

Advocates of immigration limits say this clause would be unnecessary if Congress could not otherwise control movement into the United States. After all, a common guide for reading legal documents was, “The exception proves the rule.”

As I point out in my book, The Original Constitution, both sides are missing something. They are missing the clause in the Constitution giving Congress “Power … To define and punish … Offenses against the Law of Nations.”

“The law of nations” was the usual 18th-century term for international law. It included standards of conduct among nations. But it also encompassed some rules within national boundaries. A power to “define and punish” an “offense against the law of nations” included protecting foreign ambassadors against interference, protecting safe-conduct passes — and restricting immigration.

Why have so many writers — including some constitutional law professors — missed this? One reason is that 18th-century legal terms and categories were different from those we use today. For example, a modern law book might feature a heading for “immigration law.” But in William Blackstone’s "Commentaries," the English book that served as America’s most popular legal treatise, there was no such heading. 

Instead, Blackstone addressed the topic in his chapter on the British king’s prerogative powers. When discussing safe-conduct passes, Blackstone observed that without them “by the law of nations no member of one society has a right to intrude into another ... [I]t is left in the power of all states, to take such measures about the admission of strangers, as they deem convenient.”

Blackstone relied partly on the writings of Samuel Von Puffendorf, a German international law scholar. Puffendorf’s writings were familiar to Americans. Even more popular were the writings of the Swiss lawyer, Emer de Vattel. His famous book on international law did not have a topic entitled “immigration.” But it did discuss both immigration and emigration as part of the law of nations. For example, Vattel wrote that in “Switzerland and the neighboring countries ... the law of nations ... did not permit a state to receive the subjects of another state into the number of its citizens.”

This should teach us two civic lessons. First, the Constitution did grant the federal government authority over immigration. Second, to understand the Constitution you have to do more than read the document. Because it was written mostly by lawyers for an 18th-century public well educated in legal matters, to fully understand the Constitution we have to know some fundamentals of 18th-century law.

Rob Natelson is senior fellow in constitutional jurisprudence and heads up the Article V Information Center at the Independence Institute, a free-market think tank in Denver. He was previously a constitutional law professor.


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