The San Patricios were a braye group of Catholic men who fought against the United States during the Mexican American War. They marched under the flag of Saint Patrick. They believed that what the United States was doing was wrong, and decided to switch sides and fight for Mexico in trying to stop American Imperialism.
"The Saint Patrick's Battalion aka San Patricios was a unit of several hundred immigrants and expatriates of European descent that fought as part of the Mexican Army against the United States in the Mexican-American War of 1846 to 1848. Many of the battalion's members deserted or defected from the U.S. Army. " ~ Wikipedia
Great Song about the San Patricios