Mexico–United States border

A war over where Texas ended

Texas broke away from Mexico in 1836, and the United States annexed it on 29 December 1845.^[5] The two countries disagreed about where Texas stopped. Texans claimed land all the way to the Rio Grande. Mexico said the border only reached the Nueces River, further north.^[5]

President Polk sent John Slidell to buy the disputed land along with New Mexico and California. When that failed, fighting in the contested strip gave the reason for war, which began on 13 May 1846.^[5] It ended with the Treaty of Guadalupe Hidalgo, signed on 2 February 1848.^[2] Mexico gave up about 525,000 square miles, 55% of its pre-war territory. In return the United States paid $15 million and took on up to $3.25 million of Mexico's debts to US citizens.^[5]

Article V of the treaty drew the new line. It ran up the middle of the Rio Grande from the Gulf of Mexico, then along the southern and western edges of New Mexico to the Gila River, down the Gila to the Colorado, and then along the old dividing line between Upper and Lower California to the Pacific.^[2]

“The boundary line between the two Republics shall commence in the Gulf of Mexico, three leagues from land, opposite the mouth of the Rio Grande, otherwise called Rio Bravo del Norte… from thence up the middle of that river, following the deepest channel, where it has more than one…”^[2]

The Gadsden Purchase: straight lines in the desert

The 1848 line did not last long west of El Paso. The United States wanted land south of the Gila for a southern railroad to the Pacific. It also wanted to settle a quarrel over the Mesilla Valley. In 1853 New Mexico's governor claimed the valley for the territory, and Santa Anna sent troops in response.^[6] US Minister James Gadsden negotiated a new treaty with Mexico's president, Antonio López de Santa Anna. It was signed on 30 December 1853 and, in revised form, ratified in 1854.^[6]

The treaty replaced the Gila with a set of straight lines. From the Rio Grande at latitude 31°47′N the border runs due west for 100 miles, then south to 31°20′N, west along that parallel to the 111th meridian, and then in a straight line to a point on the Colorado River 20 miles below where the Gila joins it.^[4] Those are the straight segments across New Mexico and Arizona on the map today. The United States paid $10 million for 29,670 square miles.^[4][6]

Marking the line on the ground

By the 1880s the land boundary had only 52 monuments. A joint survey under commissioners John Whitney Barlow for the US and Jacobo Blanco for Mexico worked from El Paso–Ciudad Juárez in 1891 to San Diego–Tijuana in 1894 and raised the number to 258. More were added later, for 276 in total.^[3]

On 1 March 1889 the two countries set up the International Boundary Commission to handle boundary questions, and in 1900 its life was extended indefinitely. The Water Treaty of 3 February 1944 renamed it the International Boundary and Water Commission and gave it the job of sharing out the waters of the Colorado and the Rio Grande.^[3]

A border that moves with the river

A river boundary causes a problem when the river changes course. A convention of 12 November 1884 set rules for this: in short, slow and gradual erosion moved the border with the river, but a sudden change of channel did not.^[3][7] A 1905 convention modified those rules so that the Rio Grande and the Colorado would stay the boundary.^[3]

The best-known case is the Chamizal, at El Paso and Ciudad Juárez. Over the years the Rio Grande moved south, so land that had been south of the river in 1848 ended up on its northern bank. The biggest shift came after a flood in 1864.^[8] The disputed area covered about 600 acres.^[3] Mexico argued the river had jumped to a new bed, so the border had not moved and the land was still Mexican.^[8][9] In 1911 an enlarged boundary commission, chaired by a Canadian jurist, gave the part that had built up before the 1864 flood to the United States and the rest to Mexico. The US commissioner dissented.^[7] The United States refused to accept the award.^[8]

The dispute lasted another half-century. On 18 July 1963 President Kennedy approved a settlement that gave effect to the 1911 award, and the Chamizal Convention was signed on 29 August 1963.^[10][3] The IBWC moved 4.4 miles of the Rio Grande into a new concrete-lined channel, and a net 437.18 acres passed from the US side to Mexico.^[3] More than 5,600 people, mostly Mexican Americans, had to leave their homes in El Paso.^[8] President Johnson proclaimed the settlement on 28 October 1967, and the river was turned into its new channel the next year.^[8][9]

Elsewhere the two countries simply moved the river. The Convention of 1 February 1933 straightened 155 miles of the Rio Grande through the El Paso–Juárez Valley. The Treaty of 23 November 1970 settled the remaining boundary differences and fixed the Rio Grande as the border along its whole 1,255-mile stretch.^[3]

Fences

In the early 1990s the US Border Patrol, helped by the Army Corps of Engineers, built 10-foot steel fencing along roughly 14 miles of the border near San Diego.^[11] The Secure Fence Act of 2006 called for double fencing on several stretches. A 2008 law replaced that with fencing along “not less than 700 miles” of the southwest border.^[11]

By the start of 2017, the Congressional Research Service reported, 352.7 miles of pedestrian fence and 299 miles of vehicle fencing had been built along the southwest border. On 25 January 2017 President Trump ordered the construction of “a physical wall along the southern border”.^[11]