Which country lies directly south of the United States?

Mexico shares the only southern land border of the United States, running from the Pacific coast at Tijuana and San Diego across to the Gulf of Mexico at Matamoros and Brownsville. The eastern two-thirds of that line follows the Rio Grande, which is why the boundary wanders in a way no surveyor would have drawn deliberately, while the western portion is a set of straight lines fixed by the Treaty of Guadalupe Hidalgo after the Mexican-American War and adjusted by the Gadsden Purchase a few years later. Those treaties transferred an enormous area, including all or part of California, Nevada, Utah, Arizona, New Mexico and Colorado, which is why so many place names across the American Southwest are Spanish. The river's shifting channel also created lasting disputes, since a meander that moved could leave land on the wrong side, and the two governments eventually had to negotiate specific settlements to fix them.

The same river is called the Río Bravo on the Mexican side, so the two countries use different names for the boundary they share.