That was the early stages of the Know Nothing era ..

when anti-Catholic/anti-immigrant bigotry was heavily in vogue as Irish (and German) immigration significantly increased. A few interesting facts about the San Patricios (from memory): (1) The highest rate of desertion in US military history was during the Mexican War (something well over 10%) but these guys were among the few executed; (2) both US armies (Taylor and Scott) had some of the more inept generals in our history - Twiggs, Pierce, Pillow, Quitman, among others - who were stone stupid and in some instances bigots, and immigrants were especially badly treated; (3) Scott had most - but not all - of the San Pstricios executed. Among the ones he did not execute was John Riley, the alleged ringleader.