Actually, that’s not it at all. I’m playing with the idea of writing a story in which one of the main characters is a very successful U.S. citizen who emigrated from Mexico in the 60s/early 70s. I was mainly wondering about how he would possibly have gained legal status (or, ideally, if he come here legally), and in either case, how he might have served in the military, possibly Vietnam.
The most straightforward thing would have been to have him immigrate via a relative’s petition, either a spouse or parent who was a U.S. citizen or permanent resident. In theory, people without legal immigration status are not eligible to serve in the U.S. military AFAIK, so he most likely would have had to serve after immigrating legally (or legalizing, if he initially came illegally).