One of the least known aspects of the Mexican-American War is the story of the San Patricios, a group of mostly Irish-born soldiers who ditched their US military service to go fight for Mexico. Saint Patrick’s Batalion (or Batallón de San Patricio) was organized by Jon Riley, a private in the United States military who, by popular accounts, left his US post for mass one day and never came back. Riley was one of many foreign-born Catholics who left the US military for Mexico, most likely because of mistreatment by their predominantly Protestant superior officers. One can imagine that the Irish in particular felt kinship with the Mexicans both religiously and because of their struggle against an occupying power (Ireland was still part of Great Britain at this time). Rich incentives from the Mexican government—promises of land and citizenship (most San Patricios were not US citizens, even though they served in the army)—probably didn’t hurt either.
Though the San Patricios were renowned fighters during the war, things didn’t end so well for many of them. Quite a few were captured by the US military and tried for treason and desertion. None of those men received legal representation in their trials and most were ultimately executed. The US Army even tried for years afterward to actually cover-up the existence of the Batallón de San Patricio for fear that their story might encourage future desertion. A congressional inquiry in 1915 forced the military to admit its attempted historical rewrite.
In Mexico, however, it’s a much different story. The San Patricios are still remembered as heroes and are celebrated twice a year: September 12 (the recognized date of their execution) and Saint Patrick’s Day. In 1999, a film based on the story of the San Patricios was released called One Man’s Hero. The following is clip from that movie.