I mention the Potato Famine (1845), and how many Irish people migrated to the U.S. 

Further migration to the west and the south of the country, this also happens to take place during Manifest Destiny, there was also an event of the Bible Riots in 1844 where they burned down a catholic convent in Boston. 

1846, after Texas is annexed into the U.S. in 1845, Mexico denies such annexation due to the conflicts in such territory in the 1830s, and thus war starts. 

Many of the Irish immigrants immediately upon arrival to the U.S. enlist in the army with the promise of a good pay and land. Tensions arise amongst the soldiers between Catholics and Protestants. 

According to the Smithsonian article I’m using, about 40% of the U.S. army consisted of immigrants when they entered the war.

Upon serious mistreatment from the U.S. army due to xenophobia and anti-Catholic sentiments, many of the soldiers of immigrant backgrounds (this consisted of Irish, Polish, Germans, Italians, etc.) fled or deserted, among them John Riley.  

These men joined the Mexican Army, which, by the words of Santa Anna “The Mexican nation only looks upon you as some deceived foreigners, and hereby stretch out to you a friendly hand, offer you the felicity and fertility of their territory.”

Thus those who had left the U.S. army formed the Saint Patrick’s Battalion, made up of Infantry and Artillery soldiers, complete with their own banner (it’ll be at the bottom of the post), In the article by Jaime Fogarty, he mentions the banner: The group’s banner displayed an Irish harp surrounded by the Mexican coat-of-armswith a scroll reading, “Freedom for the Mexican Republic,” and underneath the harp was the motto in  Gaelic “Erin go Brágh” (Ireland for Ever). On the other side of the banner Saint Patrick was depicted holding a pastoral staff resting on a serpent. A U.S. soldier described it as “a beautiful green silk banner [that] waved over their heads; on it glittered a silver cross and a golden harp, embroidered by the hands of the fair nuns of San Luis Potosí. 

John Riley, mentioned before, was given the rank of Brevet Major in the Saint Patrick’s. 

They fought in a series of battles important in the Mexican-American War: 

Battle of Monterrey (September 21–24, 1846) such battle happened within the city walls, with Texas rangers sent by William J. Worth, according to Mexican chroniclers, the rangers upon Worth’s orders would open fire on civilians, ransack homes, and took part in looting. The battle ended with the Mexican army to leave the city after negotiations, thus allowing for the U.S. army to take a hold of it. This battle earned the Saint Patrick’s a reputation of holding their ground. Many soldiers of the U.S. Army deserted and joined the Mexican ranks after this battle. 

Battle of Buena Vista or La Angostura (February 22–23, 1847), this was a battle with an ambiguous ending, both sides claiming victory, notably one of the fiercest known fights of the Saint Patrick’s, “Two six-pounder cannon of the U.S. Fourth Artillery were captured by the enemy due to intense fire from the San Patricio cannoneers, aided by support troops.” Several Irishmen were granted the War Cross from the Mexican Army and promoted in rank. 

A small break here noting that after La Angostura, Winfield Scott’s troops reach Mexico through sea, landing in Veracruz in order to advance to Mexico City.

Battle of Cerro Gordo (April 1847), after being sent from San Luis Potosi to Xalapa (Jalapa), the Saint Patrick’s had re-organized and were now officially called The Foreign Legion of Saint Patrick. John Riley and Santiago O’Leary were made captains of the First and Second Companies, and Francisco Moreno was made colonel and commander of the legion. Despite the efforts, this battle also resulted in an American victory. 

Battle of Churubusco (August 20, 1847), final battle of the battalion, the Saint Patrick’s and another battalion, Los Bravos were on the parapets of the convent of Churubusco. They were outnumbered, resulting in total defeat, and an estimate of 75 -85 of the men in the battalion captured and others escaped. In Fogarty’s article, according to Dr. Michael Hogan in his own book The Irish Soldiers of Mexico, “Captain Patrick Dalton of the San Patricios tore the white flag down, and General Pedro Anaya ordered his men to fight on with their bare hands if necessary”. To continue in the aftermath, “ 35 San Patricios were killed in action, O’Leary and O’Reilly were wounded and Francis O’Connor lost both legs as a result of his heroic stand against the invaders” 

What happened to the others was that they were put on trial for desertion, those before the war was declared were whipped and branded, and those who deserted after were sentenced to the gallows. Those that were only whipped and branded were forced to dig their comrade's graves. Even Francis O’Connor, who as mentioned, had lost his legs, was dragged from the hospital and was put to the gallows, Patrick Dalton, who had ripped off the flag of surrender, estimated to be 20-22 years of age, did not have a quick death. 

These actions, had of course, outraged and angered the Mexican population, condemning the Americans and the U.S. army, stating these were acts of barbarity. Oddly enough, despite the horrendous acts of the executions, more deserted the U.S. army. 

While this battalion isn’t exactly commemorated outside of Mexico, there is a ceremony that celebrates their participation in the Mexican-American War each September 12th and every March 17th. 

This is a much lesser known part of history that I never thought I’d be absolutely interested in, and here I am wanting to make an undergrad thesis out of it.