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. To the left lies San Diego, California, and on the right is Tijuana, Baja California. The building in the foreground on the San Diego side is a sewage treatment plant built to clean the Tijuana River.
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.