Saint Patrick’s Battalion in the Mexican War

Reconstruction of the battalion's flag as described by John Riley.

The United States war against Mexico saw the highest desertion rate in American military history; Saint Patrick’s Battalion included many of those deserters.

One of the most remarkable things about the Mexican War (1846–1848) was the extraordinarily high rate of desertion among the American forces. Historians differ about the exact numbers, but apparently somewhere between 8 and 13 percent of the soldiers involved deserted; a rate twice as high as during the Vietnam War, and higher than in any other foreign war fought by the United States. Most of these deserters vanished into obscurity; but a significant number organized themselves into a Mexican army company called Saint Patrick’s Battalion, since Irish immigrants were the largest single national group among them.

Why the Immigrants Deserted

The 1840s saw many European migrants arrive in the United States. The greatest number of them came from Ireland, a result of several years of poor potato harvests leading to widespread famine; but there were many immigrants from Germany, France, Italy, Spain, Poland, and other nations as well. This influx of newcomers was reflected in the composition of the armed forces: about half of the regular army were immigrants. They were not always welcomed. An anti-Catholic, anti-foreigner movement called “Nativism” had developed during the decade, and it was especially strong within the officer corps of the army. Punishments were inflicted readily on the largely immigrant, mostly Catholic enlisted men; they were often disproportionate to the offense and usually severe. One immigrant wrote that “the foreign-born soldier, especially if he happened to be Irish or German, automatically received a harsher sentence than a native American would for the same offense.” Another commented: “If a poor devil wants to be ever so religious, it’s no use of trying it here [in the army]. I suppose that’s what you call liberty of conscience in this blessed free republic of ours.”

There were other conditions prompting desertions as well. The war was not a popular one, especially in the northern states, whose inhabitants viewed it as a way for the southern states to add more territory for slaveholders. There were the usual hardships of warfare, especially in a tropical climate: heat and humidity, spoiled food, bad water, dysentery. And, for the predominantly Catholic immigrants among the troops, there was the moral difficulty of waging war against other Catholics. On top of all that, the Mexican government did all it could to encourage desertions among the Americans, offering free land, cash bonuses, and citizenship in return for service in the Mexican army.

The Short Life of Saint Patrick’s Battalion

The chief organizer of what became Saint Patrick’s Battalion was an Irish immigrant named John Riley. He was one of the first American soldiers to change sides, swimming across the Rio Grande in early April 1846, even before war was officially declared. He was made a lieutenant in the Mexican army, and recruited other Americans to join him until his force numbered over 700. Named the San Patricios because most of them were Irish, the Battalion fought fiercely at Monterrey, Buena Vista, Cerro Gordo, Churubusco, and Mexico City. Knowing that they would be executed as deserters if captured, the members of the Battalion sometimes threatened their own Mexican superiors when they attempted to surrender; at Churubusco they shot and killed two Mexicans who tried to raise a white flag.

The Mexican War was brief but intense; by some measures it was the deadliest war in American history, resulting in the deaths of 1 in every 10 American soldiers in less than two years of fighting. Most of Saint Patrick’s Battalion who survived the fighting, including John Riley, were found guilty of desertion and sentenced to be hanged. Riley’s sentence was later reduced because his desertion took place before the actual declaration of war; he and some others were instead ordered to “receive 50 lashes on their bare backs, to be branded with the letter ‘D’ for deserter, and to wear iron yokes around their necks for the duration of the war.”

The Legacy of the Battalion

John Riley and the other San Patricios, though regarded as traitors by most Americans at the time, were and are considered heroes by most Mexicans and Irish today. Various schools, churches, and streets in Mexico are named for the Battalion; and in 2004 the Mexican government presented a commemorative statue to the government of Ireland in honor of the Battalion’s service to Mexico. In the United States, the Department of the Army officially denied the existence of Saint Patrick’s Battalion until 1915, when, after a congressional inquiry, the army’s documents on the Battalion were ordered to be turned over to the National Archives. The words of John Riley during the Mexican War are worth remembering in the context of America’s many other wars since then: “Be not obscured by the prejudice of a nation, America, which is at war with Mexico…for a more hospitable or friendlier people…than the Mexicans there exists not on the face of the earth to a foriner [sic] and especially to an Irishman and a Catholic.”


  1. Howe, Daniel Walker. What Hath God Wrought: The Transformation of America, 1815–1848 (The Oxford History of the United States). New York: Oxford University Press, 2007.
  2. Stevens, Peter F. “Mexican War Traitor, Hero, and Irishman: The Three Faces of John Riley.” History News Network