Mexico–United States border crisis in the context of "U.S.–Mexico 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.

In United States constitutional theory, compact theory is an interpretation of the Constitution which asserts the United States was formed through a compact agreed upon by all the states, and that the federal government is thus a creation of the states. Consequently, under the theory, states are the final arbiters over whether the federal government has overstepped the limits of its authority as set forth in the compact. Compact theory contrasts with contract theory, which holds that the United States was formed with the consent of the people—rather than the consent of the states—and thus the federal government has supreme jurisdiction over the states. Compact theory has never been upheld by the courts.

Compact theory featured heavily in arguments by southern political leaders in the run up to the American Civil War that states had a right to nullify federal law and to secede from the union. It also featured in southern arguments opposing desegregation after the 1954 Supreme Court decision in Brown v. Board of Education. The theory also entered into the Mexico–United States border crisis of the early 2020s.

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.

The vast majority of the current border was decided after the Mexican–American War (1846–1848). Most of the border is situated along the Rio Grande River, which marks the boundary between Texas and northeastern Mexico.