There are several cities in Texas that are separated from Mexico by the Rio Grande. These cities along the border include El Paso, Laredo and Brownsville.
According to HowStuffWorks, El Paso, Laredo and Brownsville are "twin border cities" that are adjacent to several cities in Mexico on the opposite side of the Rio Grande. El Paso is adjacent to Ciudad Jurez, Laredo is adjacent to Nuevo Laredo, and Brownsville is adjacent to Matamoros. The Rio Grande was declared the southern border by the republic of Texas in 1836; this in part led to the Mexican War. In 1848, after the war, the Rio Grande became the border between the United States and Mexico thanks to the Treaty of Guadalupe Hidalgo.