How a Mexican Town Became a Birthright Citizenship Test Case for the U.S.

For decades, the United States remained unaware of a town in Mexico that, in every practical sense, belonged to the U.S. Rio Rico, founded in 1929, sat just south of the Rio Grande and an hour’s drive from the southern tip of Texas. Its residents used Mexican pesos, paid Mexican taxes, and lived under Mexican law—yet nothing about the town suggested it was anywhere but Mexico.

The issue lay in the U.S.–Mexico border, which was not where anyone thought it was. Years earlier, an American irrigation company had cut an unauthorized cutoff in the Rio Grande, creating a loop of U.S. land south of the river’s new course. Under treaties governing the boundary, artificial shifts in the river could not alter the legal border. The boundary remained fixed along the river’s original path, now a dried-up riverbed over which Rio Rico expanded. The town, founded on Mexican land, had inadvertently crossed an invisible line into the U.S.

No one noticed the error for years. It wasn’t until the 1960s that an American geography professor uncovered the discrepancy. By then, American officials faced 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 solution. A 1970 treaty restored the Rio Grande as the boundary, and in 1977, the United States formally transferred the land under Rio Rico to Mexico. The map once again aligned with reality—but the law does not shift as easily as a river.

During the decades before the correction, children were born in Rio Rico on what was legally U.S. soil. One of them, Homero Cantú Treviño, later entered the United States and faced deportation for overstaying his visa. His defense was simple: he was not an undocumented immigrant at all. He was 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 now central to Donald Trump’s push to restrict birthright citizenship: What does it mean to be “subject to the jurisdiction” of the United States?

Advocates of a narrower interpretation—primarily right-wing immigration restrictionists—argue that birth on U.S. soil alone is not enough. The Trump administration and its legal team recently made this case before the Supreme Court in Trump v. Barbara. They contended that children born in the U.S. to undocumented immigrant parents should not receive automatic citizenship because they are not fully integrated into the nation or subject to its jurisdiction. The Fourteenth Amendment, they claimed, applies only to those fully under American authority.

Rio Rico appeared to be a near-perfect test case for this argument. 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—yet its soil had produced a U.S. citizen by birth.