Saint Patrick’s Battalion: Why Irish-American Troops Defected to Mexico
The soldiers of Saint Patrick’s Battalion are regarded as national heroes in Mexico and Ireland.
March 17 is, of course, St. Patrick’s Day (known in Gaelic as Lá Fhéile Pádraig, i.e., “the Day of the Festival of Patrick”), which is actually a serious religious holiday in the Republic of Ireland in addition to being a giant drunkfest in the United States. Accordingly, speaking myself as a proud Scots-Irish-American, I am proud to present a three-part St. Patrick’s Day special series on Celtic warriors for The National Interest (granted, we already got a wee bit of a head start with our March 6, 2025, story on the Irish Air Corps jet fighter purchase plan).
We shall start by telling the story of a mostly forgotten but still noteworthy nineteenth-century military unit that actually bore the name of the Patron Saint of Ireland: El Batallón de San Patricio (Saint Patrick’s Battalion), aka the San Patricios, Los Colorados Valientes, and the Foreign Legion of Patricios, who fought on the side of Mexico during the 1846 Mexican-American War.
St. Patrick’s Battalion
The proud and patriotic service of Irish Americans—whether first-generation naturalized immigrants or stateside-born, and whether Catholic or Protestant—in wartime is well-known, especially the legendary “Fighting 69th,” so this begs the question of why a significant number of Irish immigrants to the United States ended up taking up arms against the American government on behalf of a foreign sovereign power. Frederick Wertz gives us the answer in a March 2023 article for IrishCentral:
“As they arrived in the U.S., many were pressured into joining the army – an offer they often seized when promised a good paycheck and land after the war … But life wasn’t easy anywhere for Irish immigrants at this time in history. Many found U.S. military culture very alienating and were brutally abused by fellow soldiers and superiors who hated Irish Catholics – nativist sentiment was unfortunately strong in the U.S. Army at the time … On top of that, military life only offered Protestant religious services. Once these Irish immigrants began to learn more about their enemy – Mexico printed pamphlets persuading immigrants to desert the U.S. army – some felt they had more in common with Mexicans than Americans.”
Contrast that with the American Civil War two decades later, with the famous imagery of Union Army Catholic Chaplain Fr. William Corby conferring absolution upon the Irish troops prior to the Battle of Gettysburg.
At maximum strength, Los San Patricios had a maximum strength of 200, which, as noted by military historian Brian McGinn, comprised less than 4 percent of the 4,811 Irish-born soldiers who served in the U.S. Army during the Mexican-American War; in other words, the overwhelming majority did notdesert.
Those four percenters were led by Brevet Major John Patrick Riley (1817-1850). Born Seán Ó Raghailligh, he served in the British Army before emigrating to Canada and then to the United States, arriving in Michigan and enlisting in the U.S. Army shortly thereafter. At the time of his desertion (or defection, if you prefer) in 1846, Riley was serving as an artillery officer with Company K of the 5th U.S. Infantry Regiment; from there, he and fellow defector Patrick Dalton would join the Mexican Army and form the Batallón. He was known to his fellow soldiers as “Juan Reley.”
How Well Did Saint Patrick’s Battalion Fight?
Not surprisingly at all, they fought with bravery, ferocity, and tenacity, true to the Celtic warrior spirit. Marching into battle under an emerald green banner with “Erin Go Bragh” emblazoned across it, Riley’s troops participated in a total of six engagements: the Siege of Fort Texas, the Battle of Monterey, the Battle of Buena Vista, the Battle of Cerro Gordo, the Battle of Churubusco, and the Battle for Mexico City. The battles of Buena Vista and Monterey are the same battles in which future President of the Confederate States of America Jefferson Davis commanded the 1st Mississippi Rifles, though my research for this article did not indicate that Davis and his troops ever went up against the Patricios.
However, another iconic Civil War figure, General and U.S. president Ulysses S. Grant, did go up against the Patricios at Churubusco as a young lieutenant, and he described this as the most hard-fought battle of the entire war. More than half the soldiers of the Batallón were killed in action, in part because they refused to surrender, and the fight, which ended only after hand-to-hand combat inside the walls of a Catholic convent, signaled both the last stand of these Irish-American-Mexicans and the end of the war as a whole.
The Aftermath and Legacy of Saint Patrick’s Battalion
Of the remaining Patricios who did get captured alive, they were found guilty of treachery as deserters by a military court-martial. Forty-eight of them were sentenced to death by hanging, whilst the rest were branded with the letter “D” for deserter and sentenced to severe floggings and long terms of imprisonment.
Riley was part of the contingent branded with the “D.” He was lucky enough to be able to avoid the hangman’s noose on a technicality, as he had deserted before the United States officially declared war against Mexico. Surprisingly enough, he was released and eventually rejoined the Mexican forces and even went on to serve in the regular after the war ended, attaining the rank of “permanent major.”
For nearly seventy years, the U.S. government refused to even acknowledge the existence of the San Patricio Brigade, and it wasn’t until 1915 that a congressional inquiry revealed the Mexican Army coverup of their history. By sharp contrast, as noted by the National Park Service, these soldiers are regarded as national heroes in Mexico and Ireland. In Mexico City’s San Jacinto plaza, where a plaque honors the Battalion, a squad of Mexican bagpipers plays a monthly tribute to their service.
Slainte and ¡Salud!
About the Author: Christian D. Orr
Christian D. Orr was previously a Senior Defense Editor for National Security Journal (NSJ) and 19FortyFive. He is a former Air Force Security Forces officer, Federal law enforcement officer, and private military contractor (with assignments worked in Iraq, the United Arab Emirates, Kosovo, Japan, Germany, and the Pentagon). Chris holds a B.A. in International Relations from the University of Southern California (USC) and an M.A. in Intelligence Studies (concentration in Terrorism Studies) from American Military University (AMU). He has also been published in The Daily Torch, The Journal of Intelligence and Cyber Security, and Simple Flying. Last but not least, he is a Companion of the Order of the Naval Order of the United States (NOUS). If you’d like to pick his brain further, you can ofttimes find him at the Old Virginia Tobacco Company (OVTC) lounge in Manassas, Virginia, partaking of fine stogies and good quality human camaraderie.
Image: Wikimedia Commons.
