Rio Grande in the context of "Mesilla Valley"

The Mexican–American War, also known in the United States as the Mexican War and in Mexico as the United States intervention in Mexico, (April 25, 1846 – February 2, 1848) was an invasion of Mexico by the United States. It followed the 1845 American annexation of Texas, which Mexico still considered its territory because it refused to recognize the Treaties of Velasco, signed by President Antonio López de Santa Anna after he was captured by the Texian Army during the 1836 Texas Revolution. The Republic of Texas was de facto an independent country, but most of its Anglo-American citizens who had moved from the United States to Texas after 1822 wanted to be annexed by the United States.

Sectional politics over slavery in the United States had previously prevented annexation because Texas would have been admitted as a slave state, upsetting the balance of power between Northern free states and Southern slave states. In the 1844 United States presidential election, Democrat James K. Polk was elected on a platform of expanding U.S. territory to Oregon, California (also a Mexican territory), and Texas by any means, with the 1845 annexation of Texas furthering that goal. However, the boundary between Texas and Mexico was disputed, with the Republic of Texas and the U.S. asserting it to be the Rio Grande and Mexico claiming it to be the more-northern Nueces River. Polk sent a diplomatic mission to Mexico in an attempt to buy the disputed territory, together with California and everything in between for $25 million (equivalent to $798 million in 2024), an offer the Mexican government refused. Polk then sent a group of 80 soldiers across the disputed territory to the Rio Grande, ignoring Mexican demands to withdraw. Mexican forces interpreted this as an attack and repelled the U.S. forces on April 25, 1846, a move which Polk used to convince the Congress of the United States to declare war.

Las Cruces (/lɑːsˈkruːsɪs/; Spanish: [las 'kruses]; lit. 'the crosses') is a city in Doña Ana County, New Mexico, United States, and its county seat. The population was 111,385 at the 2020 census, making it the second-most populous city in New Mexico, and the largest in both Doña Ana County and southern New Mexico. It is the principal city of the Las Cruces metropolitan statistical area, which encompasses all of Doña Ana County and had an estimated 230,000 residents in 2024. The city is also part of the larger El Paso–Las Cruces combined statistical area.

Las Cruces is the economic and geographic center of the Mesilla Valley, the agricultural region on the floodplain of the Rio Grande, which extends from Radium Springs to the west side of El Paso, Texas. The Organ Mountains, 10 miles (16 km) to the east, are dominant in the city's landscape, along with the Doña Ana Mountains, Robledo Mountains, and Picacho Peak. Las Cruces lies 225 mi (362 km) south of Albuquerque, 42 mi (68 km) northwest of El Paso, and 41 mi (66 km) north of the Mexico–United States border.

The international boundary separating Mexico and United States extends from the Pacific Ocean to the Gulf of Mexico. The border traverses a variety of terrains, ranging from urban areas to deserts. It is the most frequently crossed border in the world, with approximately 350 million documented crossings annually. Illegal crossing of the border to enter the United States has caused the Mexico–United States border crisis. It is one of two international borders that the United States has, the other being the northern Canada–United States border; Mexico has two other borders: with Belize and with Guatemala.

Four American Sun Belt states border Mexico: California, Arizona, New Mexico and Texas. One definition of Northern Mexico includes only the six Mexican states that border the U.S.: Baja California, Chihuahua, Coahuila, Nuevo León, Sonora and Tamaulipas. The total length of the continental border is 3,145 kilometers (1,954 miles). From the Gulf of Mexico, it follows the course of the Rio Grande (Río Bravo del Norte) to the border crossing at Ciudad Juárez, Chihuahua, and El Paso, Texas. Westward from El Paso–Juárez, it crosses vast tracts of the Chihuahuan and Sonoran deserts to the Colorado River Delta and San Diego–Tijuana, before reaching the Pacific Ocean.

A border wall has been built along portions of the Mexico–United States border in an attempt to reduce illegal immigration to the United States from Mexico. The barrier is not a continuous structure but a series of obstructions variously classified as "fences" or "walls".

Between the physical barriers, security is provided by a "virtual fence" of sensors, cameras, and other surveillance equipment used to dispatch United States Border Patrol agents to suspected migrant crossings. In May 2011, the Department of Homeland Security (DHS) said it had 649 miles (1,044 km) of barriers in place. A total of 438 miles (705 km) of new primary barriers were built during Donald Trump's first presidency, dubbed the "Trump wall", though Trump had repeatedly promised a "giant wall" spanning the entire border and that Mexico would "pay for the wall," neither of which were done. The national border's length is 1,954 miles (3,145 km), of which 1,255 miles (2,020 km) is the Rio Grande and 699 miles (1,125 km) is on land.

The Nueces River (/njuˈeɪsɪs/ new-AY-siss; Spanish: Río Nueces, IPA: [ˈri.o ˈnweses]) is a river in the U.S. state of Texas, about 315 miles (507 km) long. It drains a region in central and southern Texas southeastward into the Gulf of Mexico. It is the southernmost major river in Texas northeast of the Rio Grande. Nueces is Spanish for nuts, specifically pecans; early settlers named the river after the numerous pecan trees along its banks.

The Thornton Affair was a battle in 1846 between the military forces of the United States and Mexico 20 miles (32 km) west upriver from Zachary Taylor's camp along the Rio Grande. The much larger Mexican force defeated the Americans in the opening of hostilities, and was the primary justification for U.S. President James K. Polk's call to Congress to declare war. It is also known as the Thornton Skirmish, Thornton's Defeat, or Rancho Carricitos.

An oxbow lake is a U-shaped lake or pool that forms when a wide meander of a river is cut off, creating a free-standing body of water. The word "oxbow" can also refer to a U-shaped bend in a river or stream, whether or not it is cut off from the main stream. It takes its name from an oxbow which is part of a harness for oxen to pull a plough or cart.

In South Texas, oxbows left by the Rio Grande are called resacas. In Australia, oxbow lakes are called billabongs.

The Rio Grande rift is a north-trending continental rift zone. It separates the Colorado Plateau in the west from the interior of the North American craton on the east. The rift extends from central Colorado in the north to the state of Chihuahua, Mexico, in the south. The rift zone consists of four basins that have an average width of 50 kilometres (31 mi). The rift can be observed on location at Rio Grande National Forest, White Sands National Park, Santa Fe National Forest, and Cibola National Forest, among other locations.

The Rio Grande rift has been an important site for humans for a long time, because it provides a north–south route that follows a major river. The Rio Grande follows the course of the rift from southern Colorado to El Paso, where it turns southeast and flows toward the Gulf of Mexico. Important cities, including Albuquerque, Santa Fe, Taos, Española, Las Cruces, El Paso, and Ciudad Juárez, lie within the rift.

New Mexico is a landlocked state in the Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also borders the state of Texas to the east and southeast, Oklahoma to the northeast, and shares an international border with the Mexican states of Chihuahua and Sonora to the south. New Mexico's largest city is Albuquerque, and its state capital is Santa Fe, the oldest state capital in the U.S.—founded in 1610 as the government seat of Nuevo México in New Spain—and the highest in elevation, at 6,998 feet (2,133 m).

New Mexico is the fifth-largest of the fifty states by area, but with just over 2.1 million residents, ranks 36th in population and 45th in population density. Its climate and geography are highly varied, ranging from forested mountains to sparse deserts; the northern and eastern regions exhibit a colder alpine climate, while the west and south are warmer and more arid. The Rio Grande and its fertile valley runs from north-to-south, creating a riparian biome through the center of the state that supports a bosque habitat and distinct Albuquerque Basin climate. One–third of New Mexico's land is federally owned, including many protected wilderness areas, 15 national parks and monuments, and three UNESCO World Heritage Sites wholly contained within the state—the most of any U.S. state.