The Chieftans, who have been around since 1963, have just released San Patricio, with Ry Cooder's usual quality assistance, which tells the story in song of the Batallon de San Patricio which fought with the Mexican Army from 1846 to 1848.
Not many Americans supported the Mexican War or had any desire to explore the Chihuahua desert, so when tens of thousands of Irish fleeing the devastation of the Potato Famine stepped off the boat, recruiters were waiting (as they were later during the Civil War to nab arriving Germans). Being broke and landless, the appeal of pay and promised land probably looked like a rare opportunity to them. These promises were of course not honored later, the same treatment as was given black recruits during the Revolutionary War, and native Indians for centuries. There was a strong anti-immigrant nativist sentiment abroad in the land at the time, and the newly minted soldiers were probably given a rough time. As they headed south, they also started to feel more sympathy for the Catholic Mexicans than their new masters who seemed like their former British overlords, not comrades. They weren't the only ones with second thoughts, as 9000 deserted the U.S. Army during the war -- a pretty big number for a relatively small force.
Between 175 and 700 immigrant soldiers deserted to the Mexican side, mostly Irish but also a few escaped slaves and several other European nationalities, expatriate veterans looking for excitement and cash (Mexico offered land grants also, and a few survivors actually received them). Organized into an artillery unit in 1846, their ambition, fighting spirit and, especially, their previous military experience made them a formidable obstacle for Zachary Taylor's invasion force, especially his dangerous Horse Artillery. In their first action as the Batallon, in defense of Monterrey, the unit was very effective but the city surrendered. In the last, at the Churubusco Convent, they stood solidly and repelled several determined attacks, but ran out of ammunition. Disbanded at the end of the war in 1848, a few members stayed in Mexico, and they were the lucky ones: the captured San Patricios were treated as traitors and hanged, although this was illegal (only spies and those charged with atrocities are to be hanged; firing squad was specified for desertion). Of all the deserters recaptured, only the Patricios were hanged, and the Army denied this had happened until 1915.
This extraordinary story doesn't show up in histories, but a movie was released in 1999 (One Man's Hero), and on the anniversary of the executions a Mexican flag is flown in Clifden, Ireland, home of the Batallon's Major John Riley.
The Mexicans had the last laugh, since we ended up with Texas. Hope they enjoy the irony.