The United States–Mexico border is the international border between the United States and Mexico. It runs from Imperial Beach, California, and Tijuana, Baja California, in the west to Matamoros, Tamaulipas, and Brownsville, Texas, in the east, and traverses a variety of terrains, ranging from major urban areas to inhospitable deserts. From the Gulf of Mexico it follows the course of the Rio Grande (Río Bravo del Norte) to the border crossing at El Paso, Texas, and Ciudad Juárez, Chihuahua; westward from that binational conurbation it crosses vast tracts of the Sonoran and Chihuahuan Desert, the Colorado River Delta, westward to the binational conurbation of San Diego and Tijuana before reaching the Pacific Ocean.

The border's total length is 3,169 km (1,969 mi), according to figures given by the International Boundary and Water Commission. It is the most frequently crossed international border in the world, with approximately three hundred fifty million (350,000,000) crossings per year.

The nearly 2,000-mile (3,169 km or 1,969 miles) international border follows the middle of the Rio Grande—according to the 1848 Treaty of Guadalupe Hidalgo between the two nations, "along the deepest channel" (also known as the thalweg)—from its mouth on the Gulf of Mexico a distance of 2,019 km (1,255 mi) to a point just upstream of El Paso and Ciudad Juárez. It then follows an alignment westward overland and marked by monuments a distance of 858 km (533 mi) to the Colorado River, during which it reaches its highest elevation at the intersection with the Continental Divide. Thence it follows the middle of that river northward a distance of 38 km (24 mi), and then it again follows an alignment westward overland and marked by monuments a distance of 226 km (140 mi) to the Pacific Ocean.

Mexico (ⁱ/ˈmɛksɨkoʊ/; Spanish: México, IPA: [ˈmexiko] ( listen)), officially the United Mexican States (Spanish:  Estados Unidos Mexicanos (help·info)), is a federal constitutional republic in North America. It is bordered on the north by the United States of America; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of Mexico. Covering almost two million square kilometres (over 760,000 sq mi), Mexico is the fifth largest country in the Americas by total area and the thirteenth largest independent nation in the world. With an estimated population of over 113 million, it is the eleventh most populous country and the most populous Spanish-speaking country. Mexico is a federation comprising thirty-one states and a Federal District, the capital city.

In Pre-Columbian Mexico many cultures matured into advanced civilizations such as the Olmec, the Toltec, the Teotihuacan, the Zapotec, the Maya and the Aztec before the first contact with Europeans. In 1521, Spain conquered and colonized the territory from its base in México-Tenochtitlan, which was administered as the Viceroyalty of New Spain. This territory would eventually become Mexico as the colony's independence was recognized in 1821. The post-independence period was characterized by economic instability, the Mexican-American War and territorial cession to the United States, a civil war, two empires and a domestic dictatorship. The latter led to the Mexican Revolution in 1910, which culminated with the promulgation of the 1917 Constitution and the emergence of the country's current political system. Elections held in July 2000 marked the first time that an opposition party won the presidency from the Institutional Revolutionary Party.

The United States Border Patrol is a federal law enforcement agency within U.S. Customs and Border Protection (CBP), a component of the Department of Homeland Security (DHS). It is an agency in the Department of Homeland Security that enforces laws and regulations for the admission of foreign-born persons to the United States codified in the Immigration and Nationality Act. U.S. Customs and Border Protection has more sworn, armed law enforcement officers than any other agency in the United States.

Mounted watchmen of the United States Immigration Service patrolled the border in an effort to prevent illegal crossings as early as 1904, but their efforts were irregular and undertaken only when resources permitted. The inspectors, usually called "mounted guards", operated out of El Paso, Texas. Though they never totaled more than 75, they patrolled as far west as California trying to restrict the flow of illegal Chinese immigration.

In March 1915, Congress authorized a separate group of mounted guards, often referred to as "mounted inspectors". Most rode on horseback, but a few operated automobiles, motorcycles and boats. Although these inspectors had broader arrest authority, they still largely pursued Chinese immigrants trying to avoid the National Origins Act and Chinese Exclusion Act of 1882. These patrolmen were Immigrant Inspectors, assigned to inspection stations, and could not watch the border at all times. U.S. Army troops along the southwest border performed intermittent border patrolling, but this was secondary to "the more serious work of military training." Non-nationals encountered illegally in the U.S. by the army were directed to the immigration inspection stations. Texas Rangers were also sporadically assigned to patrol duties by the state, and their efforts were noted as "singularly effective".

The United States of America (commonly abbreviated to the United States, the U.S., the USA, America, and the States) is a federal constitutional republic comprising fifty states and a federal district. The country is situated mostly in central North America, where its forty-eight contiguous states and Washington, D.C., the capital district, lie between the Pacific and Atlantic Oceans, bordered by Canada to the north and Mexico to the south. The state of Alaska is in the northwest of the continent, with Canada to the east and Russia to the west, across the Bering Strait. The state of Hawaii is an archipelago in the mid-Pacific. The country also possesses several territories in the Pacific and Caribbean.

At 3.79 million square miles (9.83 million km²) and with over 312 million people, the United States is the third or fourth largest country by total area, and the third largest by both land area and population. It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries. The U.S. economy is the world's largest national economy, with an estimated 2011 GDP of $15.1 trillion (22% of nominal global GDP and over 19% of global GDP at purchasing-power parity). Per capita income is the world's sixth-highest.