As far back as the 19th century, when what had been part of Mexico became part of the U.S., people of Mexican heritage had been considered “white” according to the U.S. government. That changed in 1930 when the U.S. census bureau decided to classify them—immigrants and their descendants alike—as racially “Mexican.” The decision came at a time of hardening racial divisions in the United States: the 1920s had seen the reemergence of the KKK; the institution of the one-drop rule for African-Americans; and immigration restrictions targeting Asians, Southern Europeans and Eastern Europeans. At the time, Mexicans were the bulk of agricultural workers in the Southwest, but after the Great Depression hit they were targeted for deportation—not just immigrants, but American citizens of Mexican heritage as well. Thanks largely to the work of civil-rights activists, the census designation was changed back in 1940, but not before the impact had been made. That short period solidified the identity of Mexicans and Mexican-Americans as non-white, a perception that persists to the present.
Seems like we have been down the current path before.