Two strikingly contrasting culture groups, Latin Americans and Anglo Americans, overlap in a Borderlands that straddles the international boundary between the United States and Mexico. This overlap began with the Aztec conquest by Cortes which triggered the intermixing and miscegenation between Spaniards and Indians that produced a mestizo people and a Mexican culture. Anglo intrusion began when Mexico opened her northern frontier to trade with non-Mexicans and, in the case of Texas, even invited non-Mexicans to colonize through her empresario policy. The 1845 annexation of Texas, the 1848 Mexican Cession, together with the 1853 purchase of present-day southern Arizona, taken together, realigned Mexico's northern frontier politically, thus making Mexican Americans of the Mexican population, at least nominally. After the mid-nineteenth century, three processes reshaped the geography of the Mexican-origin people: contiguous expansion, Mexican intrusion, and peripheral attraction. Thus, Mexican-origin people became a sizable, distinctive, and diverse minority. The concentration of Mexican-origin people in the American Southwest is geographically significant because, in a demographic sense, their presence means that Latin America "shades off" into the United States, and they constitute a cultural region. Thus, people of Mexican-origin have played a major role in the Hispanicization of the United States. (NQA)