Following the defeat of the Mexican army and the fall of Mexico City in the Mexican-American War (1846-1848), Mexico ceded roughly 525,000 square miles of its territory to the United States. Bringing an end to the war, the Treaty of Guadalupe Hidalgo outlined the terms of the “Mexican Cession,” whereby Mexico transferred nearly half of its territory to the U.S., dramatically reshaping the U.S.-Mexico border.
The Treaty affected over 100,000 Mexicans of Native and Spanish descent residing in the territory, who were given one year to choose either Mexican or U.S. citizenship, or they would automatically become U.S. citizens. It is estimated that 80,000 Mexicans became U.S. citizens at that time. Mexican citizens of European ancestry were considered “white” and were granted the right to vote as U.S. citizens. Indigenous Mexicans, however, were not granted full U.S. citizenship until the 1930s. As the oldest treaty that is still in existence between the U.S. and Mexico today, it created great inequalities between the two countries that have influenced their relationship and Mexico’s development to this day.