The Irish American Soldiers Who Defected
It is a quite well known fact that at the beginning of the Mexican War, John Riley, a career military sergeant, deserted with a number of Irish-American soldiers to Mexico – and ended up forming an artillery battalion in the Mexican Army known as the San Patricios!
The reason it is understood that he and his men left the US Army is because of the harsh treatment of Irish soldiers at the hands of their officers, when at the time about half of the regular US Army, and a huge portion of its citizen soldiers, were Irish.
Irish soldiers remaining soon despised the San Patricios as traitors and deserters, and fought the San Patricios with a particularly fierce ferocity – civil wars give credence to this certain ferocity throughout history.
It was also Irish American soldiers who finally captured their erstwhile countrymen, and treated them very roughly while they awaited trial, disgusted that fellow Irishmen could so easily cast aside their new homeland and disgrace all Irishmen. The San Patricios were considered truly elite soldiers by the Mexican Army, fighting with greater vigor than other Mexican units. This can be partly explained by fact that Mexican soldiers would be treated as prisoners of war, while the San Patricios would be treated as deserters and traitors and tried accordingly.
This, of course, is exactly what happened. Those who defected after war was declared were later hanged after brief trials. Riley and many of his men who defected before the declaration of war were first whipped and branded. Riley himself never returned to the United States. He was mustered out of the Mexican Army in 1850, but his eventual fate is unknown. This would make a great story, maybe I will look into it further on behalf of my Irish Ancestry.. find US army records online