The Saint Patrick's Battalion (Spanish: Batallón de San Patricio), formed and led by John 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 1846–8. Most of the battalion's members had deserted or defected from the United States Army. The Battalion served as an artillery unit for much of the war. Despite later being formally designated as two infantry companies, it still retained artillery pieces throughout the conflict. In many ways, the battalion acted as the sole Mexican counterbalance to the recent U.S. innovation of horse artillery. The "San Patricios" were responsible for the toughest battles encountered by the United States in its in