The border between the United States and Mexico stretches for nearly 2,000 miles from the Gulf of Mexico to the Pacific Ocean and touches the states of California, Arizona, New Mexico and Texas. The Rio Grande runs along 1,254 miles of the border, but west of El Paso, Texas, the boundary lacks a natural geographic barrier except for a small stretch along the Colorado River.

Approximately 700 miles of barbed wire, chain link, post-and-rail and wire mesh fencing has been erected along the U.S.-Mexico border. The U.S. Border Patrol also utilizes thousands of cameras and underground sensors as well as aircraft, drones and boats to monitor the boundary.

After winning its independence from Spain in 1821, Mexico stretched as far north as the Oregon Territory. The secession of Texas in 1836, however, marked the beginning of the loss of Mexican territory that would become the present-day U.S. Southwest.

The War with Mexico
U.S. President James K. Polk captured the White House in 1844 on a pledge to fulfill America’s “Manifest Destiny” to stretch from the Atlantic Ocean to the Pacific Ocean. Relations with Mexico deteriorated after the United States annexed Texas in 1845. When Mexico refused an American offer to purchase California and New Mexico for $30 million, Polk dispatched 4,000 troops into land north of the Rio Grande and south of the Nueces River claimed by both countries.

Following a Mexican cavalry attack in the disputed territory on April 25, 1846, that left 16 American soldiers dead or wounded, the United States declared war on Mexico. After a series of bloody battles and sieges, American forces captured the Mexican capital in September 1847.

Under the 1848 Treaty of Guadalupe Hidalgo, Mexico formally recognized the American annexation of Texas and agreed to sell more than one-third of its territory. For $15 million and the assumption of certain damage claims, the United States purchased more than a half million square miles that would encompass all or most of the future states of California, Arizona, New Mexico, Nevada and Utah as well as portions of present-day Colorado, Wyoming, Oklahoma and Kansas.

The Establishment of the U.S.-Mexico Border
The modern border took shape following the Mexican-American War. While the Rio Grande formed the dividing line between Texas and Mexico, the border originally moved west from El Paso on a straight line to the Gila River and then on another straight line to the Pacific Ocean south of San Diego. Following the Gadsden Purchase of 1853, the borders of Arizona and New Mexico moved further south from the Gila River.

A team of surveyors, soldiers and officials from both countries staked out the border from El Paso to Tijuana. According to Rachel St. John, an associate professor of history at UC Davis and author of Line in the Sand: A History of the Western U.S.-Mexico Border, the joint boundary commission underestimated the cost and time it would take to complete the project through such an inhospitable terrain of mountains, canyons and desert. Not until the late 1850s did the boundary commission complete its work.

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U.S. Immigration Policy
There were no federal limits on immigration in the decades following the Mexican-American War as citizens from both countries passed freely across the border. It was Chinese immigrants, not Mexicans, that American authorities and vigilante groups first sought to keep from illegally crossing its southern border after the passage of the Chinese Exclusion Act of 1882. “One of the ways that immigrants from China would try to get across the border is to learn a few words of Spanish and disguise themselves as Mexican,” St. John says.

“Restrictions on the movement of Mexican citizens were not particularly enforced by the U.S. government until the decade of the Mexican Revolution in the 1910s when large numbers of refugees came to escape the war and there was a large demand for Mexican labor,” St. John says. Following Mexican revolutionary Pancho Villa’s deadly raid on Columbus, New Mexico, in 1916 and the subsequent publication of the Zimmerman Telegram proposing a World War I military alliance between Mexico and Germany, the United States tightened border security and deployed soldiers to patrol the boundary along with the Texas Rangers and government-sanctioned “home guards.”

According to St. John, the U.S. Bureau of Animal Industry erected the first fence along the frontier in 1909 to stop the trans-border movement of cattle. Border towns erected fences during the 1910s, but less as a physical barrier to entry than to denote the boundary line and channel people into designated crossing points. The United States began the installation of border fences to restrict the movement of unlawful immigrants and drugs in 1993 when President Bill Clinton mandated the construction of a 14-mile barrier between San Diego and Tijuana. The Secure Fence Act of 2006 authorized the construction of 700 miles of border fencing and vehicle barriers, which was completed in 2011.

Future Plans for the Border
Approximately 11.6 million Mexican immigrants resided in the United States in 2016, about half of them in the country illegally, according to Pew Research Center estimates. The centerpiece of President Donald Trump’s immigration plan is the construction of an “impenetrable, physical, tall, powerful, beautiful, southern border wall,” but the project faces funding, environmental and eminent domain obstacles.

While Trump asserts the construction of a new 1,000 miles of wall as high as 55 feet tall through remote, mountainous terrain can be built for $18 billion, an analysis published in MIT Technology Review estimates the cost to be $40 billion. The Mexican government stated that it would not pay for the wall’s construction, as Trump repeatedly pledged during the 2016 presidential campaign, and Congress contributed only $1.6 billion to the project in March 2018.

In April 2018, President Donald Trump ordered National Guard troops deployed to the border until further progress is made on construction of the wall. The move was not unprecedented as his predecessors George W. Bush and Barack Obama also sent the National Guard to assist with border security.