The answer to this question is that the Rio Grande is the river that forms a natural border between the United States and Mexico. Of course, this river does not form the entire border between these two countries. In addition, it was not always a part of the border.
The United States did not get control of all the land north of the Rio Grande until the 1840s. Before that, the lands bordering the river were part of Mexico. Between 1846 and 1848, the US and Mexico fought a war that the US brought about because it wanted territory that belonged to Mexico. The treaty that ended the war gave the US control over Texas and other areas north of the Rio Grande.
The Rio Grande only forms part of the border between the US and Mexico. In fact, it only forms the border between the two countries in the state of Texas. After that, the border is a land border that is an arbitrary line.
The Rio Grande River forms part of the natural border between the United States and Mexico. This river forms the border between the United States and Mexico, but only on the border with Texas. The rest of the United States-Mexican border is a land border stretching from New Mexico to California.
There was a dispute over which river originally formed the border between the United States and Mexico in Texas. The Mexicans said it was Nueces River, which would have made Texas smaller than it currently is. This border dispute was one of the factors that led to the war with Mexico in 1846. When we moved our troops into the land between the Nueces River and the Rio Grande River, Mexico attacked us. This led to the start of the war with Mexico. As a result of this war, the Rio Grande River became the border in Texas between the United States and Mexico.