The Saint Patrick's Battalion (, later reorganized as the Foreign Legion of Patricios) was a unit of 175 to several hundred (accounts vary) immigrants and expatriates of European descent who fought as part of the Mexican Army against the United States in the Mexican–American War of 1846–48. Formed and led by John Riley, the battalion's members included many who 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 were responsible for the toughest battles encountered by the United States in its invasion of Mexico, with Ulysses S. Grant remarking that "Churubusco proved to be about the severest battle fought in the valley of Mexico".