Zev Chafetz’s profile of San Antonio Mayor Julian Castro is interesting throughout and features this interesting historical tidbit:
Historically, Mexican-Americans have generally been considered “white” in Texas; they served in white units of the segregated military, including the National Guard, and were allowed, during the Jim Crow years, to marry white (but not black) partners. In the early ’40s, the Texas Legislature even passed a “Caucasian Race Resolution,” which affirmed their status as white.
If you read the text of the resolution it seems that the issue here is that the white status of Texans of Mexican origin was a contested point. The Texas legislature, in part out of concern for warm relations with Latin American countries during wartime, passed what was essentially a civil rights law banning discrimination against Hispanics.