Saint Patrick's Battalion flag (1846-1848), Mexico Sticker

The Saint Patrick's Battalion (Spanish: Batallón de San Patricio), later reorganized as the Foreign Legion of Patricios, was a Mexican Army unit which fought against the United States in the Mexican–American War. Consisting of several hundred mostly Irish and other Catholic European expatriates and immigrants, including numerous men who had deserted or defected from the United States Army, the battalion was formed and led by Irishman John Riley. It served as an artillery unit for much of the war, and despite later being formally designated as an infantry unit of two companies, the battalion continued to operate artillery pieces throughout the conflict. The San Patricios participated in many of the bloodiest battles during the American invasion of Mexico, with Ulysses S. Grant remarking that "Churubusco proved to be about the severest battle fought in the valley of Mexico". Composed primarily of Irish immigrants, the battalion also included German, Canadian, English, French, Italian, Polish, Scottish, Spanish, Swiss and Mexican soldiers, most of whom were Catholic. Several native-born Americans were in the ranks, including fugitive slaves from the Southern United States. Only a few members of the battalion were U.S. citizens. The Mexican government printed propaganda in different languages to entice immigrants serving in the United States Army to switch sides and offered incentives to foreigners who would enlist in its army, including being granted citizenship, being paid higher wages and generous land grants. U.S. Army regiments which had members defect included 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 San Patricios are honored by Mexican and Irish people.