The US-Mexico border is the 3,145 km (1,954 mile) boundary between the United States and Mexico, running from the Pacific Ocean at Tijuana and San Diego to the Gulf of Mexico at Matamoros and Brownsville. For 2,020 km of that distance it follows the Rio Grande, known in Mexico as the Río Bravo del Norte; the western 1,125 km is a surveyed land line across desert and mountains. It separates four US states, California, Arizona, New Mexico and Texas, from six Mexican ones, Baja California, Sonora, Chihuahua, Coahuila, Nuevo León and Tamaulipas. It matters because it is the most frequently crossed border in the world, with roughly 350 million documented crossings a year, and because in 2026 it is again the centre of an argument over whether a wall should run along all of it.
A Wall Being Finished in 2026
When Donald Trump began his second term on 20 January 2025, he declared a national emergency at the southern border, revived the "Remain in Mexico" policy and pledged to finish the barrier. Before that date, the existing wall that met Border Patrol operational requirements came to about 644 miles (1,036 km) of primary wall and 75 miles of secondary wall. In October 2025 the project was renamed the "Smart Wall" to reflect the sensors and detection technology layered onto it. In June 2026 the commissioner of US Customs and Border Protection (CBP), Rodney Scott, said the primary wall would be complete by the end of 2027 and the whole system by July 2028. On 18 September 2026 CBP was reported to be building an average of 12 miles of barrier a week.
The wall is being finished against a sharp change in the numbers. Between January 2021 and January 2024, more than 7.2 million migrants were encountered at the border, most of them from Guatemala, Honduras, El Salvador and Venezuela. By February, March and April 2025, monthly apprehensions had fallen to about 8,300, 7,200 and 8,400, more than 90 percent below the same months of 2024. No full year since the late 1960s has averaged fewer than 9,000 a month. The drop has been attributed to the militarization of the border and to deportations inside the United States. The crossings that do happen remain dangerous: the International Organization for Migration recorded 686 migrant deaths and disappearances on the border in 2022, making it the deadliest land migration route in the world, with deaths attributed to heat exposure, drowning and dangerous terrain.
Barriers are older than this argument. The first US fence was a barbed-wire cattle fence in Ambos Nogales, built between 1909 and 1911; Mexico put up its own wire fence there in 1918. The US Border Patrol was created in 1924. In the 1990s, Operation Gatekeeper in California, Hold-the-Line in Texas and Safeguard in Arizona added fencing made largely of surplus Vietnam War helicopter landing mats. The Real ID Act of 2005 let the Secretary of Homeland Security waive other laws to speed construction, and 438 miles (705 km) of new primary barrier went up during Trump's first term. What is new in 2026 is the aim of closing the gaps entirely.
How the 1848 Treaty of Guadalupe Hidalgo Drew the Line
The border exists in its current place because of a war. Spain and the United States had fixed a frontier in the Adams–Onís Treaty of 1819, and independent Mexico confirmed it in the Treaty of Limits of 1828. Texas broke away in 1836 and was annexed by the United States in 1845, and the two countries still disagreed over whether Texas ended at the Nueces River or the Rio Grande. The Mexican–American War followed in 1846. The Treaty of Guadalupe Hidalgo, which ended it in 1848, cost Mexico more than 2.5 million square kilometres, about 55 percent of its territory: all of present-day California, Nevada, Utah and Arizona north of the Gila River, plus most of New Mexico and parts of Colorado, Wyoming, Kansas and Oklahoma.
The treaty put the eastern boundary "along the deepest channel" of the Rio Grande, and a joint United States and Mexican Boundary Survey then marked the rest on the ground. The International Boundary Commission, created in 1889 and renamed the International Boundary and Water Commission (IBWC) in 1944, still maintains it, with a US section in El Paso and a Mexican section in Ciudad Juárez.
The Gadsden Purchase and the Straight Lines Across the Sonoran Desert
Five years after the war, the United States wanted flatter land south of the Gila River for a southern transcontinental railway. James Gadsden, the US minister to Mexico, and President Antonio López de Santa Anna signed the treaty at the end of December 1853, and it took effect on 8 June 1854. For 10 million dollars the United States acquired about 76,800 km² of what is now southern Arizona and southwestern New Mexico, including the sites of Yuma, Tucson, Nogales and Douglas. The Southern Pacific finished the railway in 1881–1883. It was the last large addition to the contiguous United States, and it produced the geometric western border that exists today.
That border begins at an initial point one marine league south of San Diego Bay. It runs 227 km in a straight line east to the Colorado River below its junction with the Gila, follows the Colorado south for 39 km, and then runs 859 km along a chain of straight lines: a diagonal to 31°20′N at the 111th meridian, east along that parallel, north along a meridian, and east again along 31°47′N until it meets the Rio Grande at El Paso and Ciudad Juárez. The border also extends 29 km into the Pacific and 19 km into the Gulf.
When the Rio Grande Moves: Chamizal and the 1970 Boundary Treaty
A river border has a problem that a surveyed line does not: the river moves. The treaties of 1848 and 1884 fixed the boundary in the middle of the Rio Grande regardless of changes in its channel, which worked until the river jumped. After a flood in 1864 the Rio Grande shifted south at El Paso, leaving a district of Ciudad Juárez called El Chamizal on the US bank. The United States governed it; Mexico never gave up its claim. The dispute ran for a century until the Chamizal Treaty of 1963, under which the United States returned 630 acres of south El Paso, formally handed over on 28 October 1967.
Other fixes followed a pattern. The Banco Convention of 1905 let the IBWC reassign "bancos", pieces of land cut off by sudden changes in the river's course, and it was applied on 37 dates between 1910 and 1976, always to small, uninhabited parcels. The Rio Grande Rectification Treaty of 1933 straightened the river through the El Paso–Juárez valley, with each country ceding the same area. The Boundary Treaty of 1970 settled every remaining uncertainty along the river, and in its first application, on 24 November 2009, the United States ceded six islands to Mexico while Mexico ceded three islands and two bancos in return.
Sharing the Rio Grande Under the 1944 Water Treaty
The river is also a shared water supply, which gives the border a second dispute beside migration. The Water Treaty of 3 February 1944 divides the Rio Grande below Fort Quitman and the Colorado River. On the Colorado, Mexico is guaranteed 1.5 million acre-feet a year. On the Rio Grande, the United States receives one-third of the flow from six named Mexican tributaries, including the Río Conchos, and that third may not average less than 350,000 acre-feet a year over each five-year cycle. Water rights in the Rio Grande Valley have continued to cause tension between farmers on the two sides of the river. The treaty also led to jointly built dams on the river and to Minute 242 of 1973, which dealt with salty water reaching Mexico through the lower Colorado.
For the river's full course from Colorado to the Gulf, see our Rio Grande river guide.
San Ysidro, Nogales and 350 Million Crossings
Most of the traffic across the border is legal and daily. About 12 million people live in the counties and municipios along the line: 23 US counties face 37 Mexican municipalities, often as paired cities. The San Ysidro Port of Entry between San Diego and Tijuana, where Interstate 5 ends, handles roughly 50,000 vehicles and 25,000 pedestrians a day. Its users include commuters who live in Tijuana and work in San Diego, and US citizens who live in Mexico and go to school in the United States. Waits of about an hour are normal.
Further east, Ambos Nogales, "both Nogales", is split by a steel border barrier, 97 km (60 miles) south of Tucson; the two cities kept a shared identity even as the barrier rose. At the Pacific end, Friendship Park, a half-acre space inside Border Field State Park that Pat Nixon inaugurated in 1971, was the only place on the whole border run as a meeting point by both countries, and families once talked through the mesh there. It has been closed during wall reconstruction. The first international bridge on the border was the Brownsville & Matamoros International Bridge of 1910; its original span still stands, and locals call it the Puente Viejo, the Old Bridge. Mexico's other land borders, with Guatemala and Belize, are covered in our guide to the countries that border Mexico, and the calmer frontier at the other end of the United States is described in our piece on the US-Canada border.