John Patrick Riley was an Irish soldier in the British Army who emigrated to the United States and subsequently enlisted in the United States Army. During the Mexican–American War of 1846–1848, Riley led a number of other Defectors in the ranks who defected to Mexico, where they formed the Saint Patrick's Battalion in the Mexican Army.

The Saint Patrick's Battalion, 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 between 175 and several hundred mostly 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".