During the Mexican-American War of 1846–1848, Irish, German, and other Catholic soldiers deserted the U.S. Army and joined the Mexican side. In November 1846, following the orders of General Antonio López de Santa Anna, hundreds of deserters joined a new artillery company, initially called Voluntarios Irlandeses, organized by John Riley (known as Juan O'Reilly in Mexico), an experienced officer born in Clifden, County Galway. Riley had served in the British army as a noncommissioned officer and in 1843 emigrated to Canada and then to the United States. Two years later he enrolled in the Fifth U.S. Infantry together with several other Irish soldiers.
From the beginning of the conflict, the Mexican army actively induced U.S. Catholic soldiers to defect to the enemy, characterizing the war as a religious struggle between Catholics and Protestants. Pamphlets circulated among the Irish and German soldiers that included a promise of promotions and land grants. Bigotry against Catholic soldiers was common among nativist U.S. officers, an attitude that further encouraged desertion, though of the 4,811 Irish-born soldiers serving in the U.S. Army, only 200-800 joined the San Patricios.
As a battery of artillery, the San Patricios first fought from September 19 to 24, 1846, in the Battle of Monterrey. The Mexican forces were defeated, but the San Patricios were granted safe passage to Saltillo by the U.S. commander, Zachary Taylor. On February 23, 1847, the battalion fiercely engaged at the Battle of Buena Vista, where it served with distinction under a banner of green silk with the image of St. Patrick. Santa Anna ordered a general retreat and the San Patricios marched to San Luis Potosí. They also distinguished themselves at Cerro Gordo (April 17 and 18, 1847) and at the decisive Battle of Churubusco (August 20, 1847), where they were decimated and the survivors made prisoners.
Between September 12 and 18, 1847, forty-seven were hanged and fifteen were whipped and branded with the letter "D" as deserters, including the leader, John Riley. The remainder of the San Patricio battalion was dispersed in 1848 by the Mexican president, José Joaquín de Herrera. The story of the San Patricios generated a significant amount of historical and fictional narratives in the form of books and films. Riley and his men were considered heroes in Ireland and in Mexico, where a street in Churubusco was named Mártires Irlandeses, but the battalion's existence put Irish Americans in an uncomfortable position within the larger society in the United States.
Cox, Patricia. Batallón de San Patricio. Mexico City: Editorial Stylo, 1954.
Hogan, Michael. The Irish Soldiers of Mexico. Mexico City: Fondo Editorial Universitario, 1997.
Stevens, Peter F. The Rogue's March: John Riley and the St. Patrick's Battalion. Washington, DC: Brassey's, 1999.