The dyadic racial thinking of white southerners and northerners encountered some challenges in the mid-nineteenth century as European whites began their westward march across the continent. In the trans-Mississippi West whites encountered Mexicans in the present-day states of Texas, New Mexico, and California. From their first encounters, Anglos (the term used by Mexicans for white Americans) did not regard Mexicans as  blacks, but they also did not regard them as white. Neither black nor white, Mexicans were usually regarded as a degraded “mongrel” race, a mixture of Indian, Spanish, and African ancestry, only different from Indians and Africans in the degree of their inferiority to whites. Indeed, many whites considered Mexicans inferior to Indians and Africans because Mexicans were racially mixed, a hybrid race that represented the worst nightmare of what might become of the white race if it let down its racial guard. Where whites encountered groups who were neither black nor white, they simply created other racial binaries (Anglo Mexican; white Chinese, and so forth) to maintain racial hierarchies, while the quality that made whites superior–their “whiteness”–assumed a kind of racelessness, or invisibility, as they went about reaping the spoils of racial domination.
Neil Foley, "Partly Colored or Other White: Mexican Americans and Their Problem with the Color Line," in Beyond Black and White: Race, Ethnicity, and Gender in the U.S. South and Southwest
, ed. Stephanie Cole and Alison M. Parker, 123-144 (College Station: Texas A&M University Press, 204), 124-125.