With the signing of the Treaty of Guadalupe-Hidalgo (1848) and the Treaty of La Mesilla (1853), the current 1,958-mile (3,152-km) border between Mexico and the United States was officially established. Under these treaties, Mexico lost almost half of its territory to the United States and about 1 percent of its population residing in those areas (Gonzáles, 1999). Mexicans who lived in the territory that now belonged to the United States were given the option of moving south to Mexico or keeping their property and becoming American citizens. Approximately 75,000 out of 100,000 Mexicans in the territory decided to remain in what became the American Southwest (Monto, 1994: 27). Although this population did not actually migrate to the United States, it can be considered the first generation of Mexican immigrants in the country as a consequence of the new territorial division.

With limited exceptions, Mexico's policies toward emigration were generally passive, based on the idea that population movements should not be controlled by the government, and the fact that emigration provided a temporary solution to the economic and political problems in the country. Attempts to manage these movements in certain periods proved unsuccessful. The Mexican government limited most of its activities regarding the population in the United States to consular protection, which varied according to each consulate and to specific situations faced by Mexican migrants.