The Saint Patrick’s Battalion (Spanish: Batallón de San Patricio), formed and led by Jon Riley, was a unit of 175 to several hundred immigrants (accounts vary) and expatriates of European descent who fought as part of the Mexican Army against the United States in the Mexican-American War of 1846 to 1848. Most of the battalion’s members had deserted or defected from the U.S. Army. Made up primarily of ethnic Irish and German Catholic immigrants, the battalion included Canadians, English, French, Italians, Poles, Scots, Spaniards, Swiss, and native Mexicans, most of whom were Roman Catholics. Disenfranchised Americans were in the ranks, including escaped slaves from the American South. The Mexican government offered incentives to foreigners who would enlist in its army: granting them citizenship, paying higher wages than the U.S. Army and the offer of generous land grants. Only a few members of the Saint Patrick’s Battalion were actual U.S. citizens.
Members of the Battalion are known to have deserted from U.S. Army regiments including: the 1st Artillery, the 2nd Artillery, the 3rd Artillery, the 4th Artillery, the 2nd Dragoons, the 2nd Infantry, the 3rd Infantry, the 4th Infantry, the 5th Infantry, the 6th Infantry, the 7th Infantry and the 8th Infantry.
The Battalion served as an artillery unit for much of the war. Despite later being formally designated as infantry, it still retained artillery pieces throughout the conflict. In many ways, the battalion acted as the sole Mexican counter-balance to U.S. horse artillery.