realistically, it would be a us invasion of mexico. chicanos have always served in the military; just give dudes 50k cash for enlisting, waive all medical history not obviously visible at meps, and then toss them another 10k if they are fully bilingual. but the military is simply too stupid to do that. they will insist on scraping the bottom of the barrel, mistakingly assuming that trailer trash who were never taken to the doctor are healthier than a gigachad that pretended to have asthma when he was 9.
United States