<P> During the late - 1830s and early - 1840s , the river marked the disputed border between Mexico and the nascent Republic of Texas ; Mexico marked the border at the Nueces River . The disagreement provided part of the rationale for the US invasion of Mexico in 1846 , after Texas had been admitted as a new state . Since 1848 , the Rio Grande has marked the boundary between Mexico and the United States from the twin cities of El Paso , Texas , and Ciudad Juárez , Chihuahua , to the Gulf of Mexico . As such , crossing the river was the escape route used by some Texan slaves to seek freedom . Mexico had liberal colonization policies and had abolished slavery in 1828 . </P>
the part of mexico north of the rio grande used to be called
Republic of Texas