After the United States ended its war with Mexico in 1848, the countries remained on shaky terms. Both countries had claims to the Mesilla Valley, Mexico alleged that the U.S. had not honored all of its agreements in the Treaty of Guadalupe Hidalgo, and Americans frequently crossed the border into Mexico in an attempt to gain territory, according to the U.S. Office of the Historian. In an attempt to resolve this tension, the U.S. negotiated what is today known as the Gadsden Treaty, named after the U.S. Minister to Mexico at the time, which, among its provisions, expanded the U.S.'s southern border by purchasing $10 million worth of land from Mexico. The U.S. hoped to use its new purchase to build a southern line of its transcontinental railroad.
The border is slanted so Mexico could keep its western seaports and a land bridge to the Baja California peninsula (via Pima County Public Library). But an urban legend maintains the border is slanted because surveyors walked from Nogales to the nearest saloon, in a city to the northwest.
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