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.