For decades, the United States didn’t realize it had a town in Mexico. Rio Rico, which sits just south of the Rio Grande and an hour’s drive from the southern tip of Texas, was founded in 1929 and functioned as part of Mexico in every practical sense. Its residents bought goods with Mexican pesos, paid Mexican taxes, and were subject to Mexican laws. Nothing about Rio Rico suggested that it belonged anywhere else.
The problem was that the U.S.–Mexico border wasn’t where anyone thought it was.
Years earlier, an American irrigation company had cut an unauthorized cutoff in the Rio Grande, leaving a loop of U.S. land south of its new course. Under treaties governing the boundary, artificial shifts in the river could not move the border. The legal boundary between the countries remained the same, and now cut through a dried-up riverbed—over which Rio Rico then expanded. So while the town was founded on Mexican land, its growth inadvertently moved it across the invisible border.
No one noticed for years. When the error was finally uncovered by an American geography professor in the 1960s, American officials were forced to confront a peculiar reality: A town that had long functioned as part of Mexico was, in fact, straddling the border.
The two governments eventually negotiated a fix. A 1970 treaty restored the river as the boundary, and in 1977, the United States formally transferred the land under Rio Rico to Mexico. The map once again matched the lived reality—but the law does not move as easily as the river does.
In the decades before the correction, children had been born in Rio Rico on American soil. One of them, Homero Cantú Treviño, later entered the United States and faced deportation for overstaying his visa. His defense was straightforward: He was not an undocumented immigrant at all, but a U.S. citizen by birth.
The Fourteenth Amendment guarantees citizenship to everyone born on American soil and subject to American jurisdiction: “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.” Cantú’s claim forced the government to confront a question that now sits at the center of Donald Trump’s attempt to restrict birthright citizenship: What does it mean to be “subject to the jurisdiction” of the United States?
For advocates of a narrower interpretation—right-wing immigration restrictionists, in the main—birth on U.S. soil is not sufficient. That’s the case that the president and his legal team made recently before the Supreme Court in Trump v. Barbara: Children born in the U.S. to undocumented immigrant parents shouldn’t get automatic citizenship because they aren’t really integrated into the nation and subject to its jurisdiction. The Fourteenth Amendment applies only to those fully subject to American authority, they argued.
Rio Rico would seem to have provided a near-perfect test case for that position. The town was governed in practice by Mexico. Its residents lived under a different sovereign, and the U.S. exercised no real authority over them. If “jurisdiction” is understood as something more than a formal abstraction, it is hard to see how it applied here. And yet, the U.S. declined to take that path.
Cantú’s case made its way through the immigration system. The Board of Immigration Appeals supported him only on narrow grounds. But in reviewing the case, President Jimmy Carter’s attorney general, Griffin Bell, went further. He acknowledged the unusual circumstances but declined to draw a limiting principle from them. Birth on U.S. soil remained the decisive fact. Cantú was granted citizenship, as was everyone else from Rio Rico who could prove birth on U.S. soil.
The episode presented an unusually clean opportunity to redefine the scope of birthright citizenship. If there were ever a case in which the U.S. might have insisted that geography alone was insufficient, this was it. The gap between sovereignty and governance was clear and undeniable, but the government chose not to draw that line.
The reason is not hard to see. Once introduced, such distinctions are difficult to contain. If jurisdiction depends on the degree of control exercised by the state, then every marginal case invites a challenge. How much control is enough? What kinds of authority count? And how are those judgments to be made in situations where the facts are less clear than they were in Rio Rico? These, no doubt, are some of the questions the Supreme Court is wrestling with as it decides how to rule on Trump v. Barbara in the next month or two.
The appeal of restricting birthright citizenship lies in its promise to align legal status with ideas about allegiance and authority. But as Rio Rico demonstrates, reality is messier. Sovereignty can be formal or practical, jurisdiction can be partial, and the relationship between the two can shift over time.
So a simple rule has its advantages. For most of American history, that rule has been that birth on U.S. soil confers citizenship. That sometimes produces results that can seem counterintuitive in edge cases. But it has the virtue of clarity. And it avoids the need to adjudicate degrees of belonging.
