In 1868, the Republican Party, then just 14 years old, produced what remains, 156 years later, its greatest accomplishment: the 14th Amendment. Donald Trump vows to change the long-standing interpretation of an important provision of it, and to do so unilaterally, “on day one,” by an executive order. Trump calls the interpretation he rejects “ridiculous,” but some intelligent and informed people also question it. They, too, are mistaken.

The issue is birthright citizenship. The amendment’s first sentence says: “All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the state wherein they reside.” The question is whether birthright citizenship is a constitutional mandate or a mere policy, albeit one sanctified by more than a century of practice. The correct answer is: a mandate.

The debate turns on the five-word phrase “subject to the jurisdiction thereof,” which some eminent scholars have described as “opaque” and “cryptic.” The amendment’s legislative history, however, clarifies the meaning of the phrase. It endorsed an idea that the Supreme Court in 1898 would call “the ancient and fundamental rule of citizenship by birth within the territory.”

This endorsement was primarily for the benefit of African Americans: The citizenship clause repudiated the Supreme Court’s worst decision, Dred Scott v. Sandford, which in 1857 held that African Americans could not be citizens. The clause should, however, also be construed in the context of the American premise, which rejects the pre-modern politics of blood and tribalism. The premise is:

Rights-bearing individuals are society’s molecular units. Their natural rights precede governments, which, as the Declaration of Independence proclaims, are instituted to “secure” those rights. Individuals are endowed with a right to recognition of these rights regardless of where birth occurs. Birth on U.S. soil is entry into the community of persons subject to U.S. laws. The citizenship clause’s “subject to the jurisdiction” language was intended to exclude only a few categories of people — e.g., the children of foreign diplomats and tribal Native Americans (to whom Congress extended citizenship in 1924). Congress in 1868 could not, in expected application of the phrase, have contemplated the exclusion of children of illegal immigrants. This is because in 1868 there were none: Until 1875, there were no national immigration laws to violate.

Today, illegal immigrants are prosecuted for breaking laws besides those concerning entry into the country: Those here illegally are “subject to” U.S. “jurisdiction.” (Here “jurisdiction” is a synonym for “authority.”) Illinois’s Lyman Trumbull, chairman of the Senate Judiciary Committee, said subject to U.S. jurisdiction meant not “owing allegiance to anybody else.”

Allegiance to the United States can be assumed, and is generally high, among immigrants whose (often arduous and dangerous) journeys from other countries constitute renunciations of allegiance to those countries. But allegiance is beside the point; the point is jurisdiction. In 2015, James C. Ho, now a judge on the U.S. Court of Appeals for the 5th Circuit, noted: In the Senate debate on the citizenship clause, some questioned its wisdom, “but no Senator disputed the meaning of the amendment with respect to alien children.”

In 2016, Trump campaigned against birthright citizenship, and as president he said he was contemplating an executive order reinterpreting the citizenship clause, but he never acted on this threat. A Trump executive order would emulate Barack Obama’s “I’ve got a pen and I’ve got a phone” theory whereby he rewrote immigration law, and Joe Biden’s attempts to spend, through unilateral student loan forgiveness — without Congress’s approval — more than $400 billion. The Obama-Biden-Trump theory is that presidents have the power to act on matters they deem urgent if Congress does not act when and how presidents desire. Progressives ideologically hold, and Trump opportunistically holds, that presidential highhandedness in pursuit of virtue is no vice.

Writing in National Affairs in 2018, Peter H. Schuck and Rogers M. Smith, of Yale and the University of Pennsylvania, respectively — both “strongly favor even more legal immigration than the U.S. now accepts” — argued that birthright citizenship is “a legitimate political and policy question” that Congress should settle. It actually is, however, settled by the legislative history of the citizenship clause, and by more than a century of practices by all three branches of government.

Congress could, as a prophylactic measure against nativist animus, codify it. But this would wrongly imply that the settled interpretation is dubious. Congress controls naturalization policies, but the Constitution, by the citizenship clause, puts birthright citizenship above the vicissitudes of politics.

Reach George Will at georgewill@washpost.com.