Chamizal
Location of the former US-Mexico border at the Chamizal National Memorial in El Paso, Texas. After the Mexican-American War, the border changed as the Rio Grande River shifted, this didn't work for either country. In 1963, after more than a century of this, a formal border was built. If interested, here's the full story: https://www.nps.gov/nr/travel/cultural_diversity/Chamizal_National_Memorial.html