By the time you read this, the US Court of Appeals for the Ninth Circuit may have handed down a ruling for or against president Donald Trump’s executive order banning travel and immigration from seven countries. Two states (Washington and Minnesota) are suing to kill that order.

Andrew Napolitano — a prominent constitutionalist and libertarian commentator, not to mention a former New Jersey Superior Court judge — writes in Reason that the states don’t have legitimate standing to sue. Why? Because the Constitution provides for quite a bit of presidential latitude on foreign policy.

I’ll explain why Judge Napolitano is wrong on the details momentarily, but first let’s get one thing out of the way: Immigration is not a foreign policy matter. Foreign policy relates to matters outside the United States and to relations between US government and other governments around the world. Immigration relates to individuals wishing to enter and possibly reside in the United States. It is therefore a matter of domestic, not foreign, policy.

It’s also a matter constitutionally reserved to the states, which is where Judge Napolitano really steps in it. He hangs his argument for the order and against the states’ legal standing on the fact that “[a] 1952 federal statute permits the president to suspend the immigration status of any person or group whose entry into the United States might impair public health or safety or national security.”

But that statute is plainly unconstitutional, for the same reason that the states have standing. Why? Because per Article I, Section 9 of the Constitution, “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 the Year one thousand eight hundred and eight …”

Article V of the Constitution forbids amending that provision prior to 1808, and no amendment to it has been proposed or ratified since that time. Congress scrupulously observed that restriction for nearly a century. As with many restrictions on federal power, it eventually got ignored. But it’s still “the supreme law of the land.”

The Constitution doesn’t enumerate a federal power to regulate immigration. In fact it clearly and unambiguously reserves that power to the states. That makes the statute Judge Napolitano references unconstitutional, and the executive order hinging on it void. Obviously states have standing to sue when the federal government usurps a power the Constitution reserves to them.