The dispute surrounding assigning the border at the Río Grande or at Nueces River, coupled with the U.S. annexation of Texas in 1845, set the Mexican-American War into motion. This slice of land between the Río Grande and the Nueces River is called the Trans-Nueces, which you can see in the middle of the two yellow lines in the center of the map on the left. Lasting from 1846-1848, the Mexican-American War ended in the Mexico-U.S. border being set at the Rio Grande and the signing of the Treaty of Guadalupe Hidalgo. As part of the Treaty, Mexico lost a devastating 55% of its land to the U.S., giving both countries the border we recognize today. Well, for the most part.
