U.S. regiments members of the battalion are known to have deserted from include the 1st Artillery, the 2nd Artillery, the 3rd Artillery, the 4th Artillery, the 2nd Dragoons, the 2nd Infantry, the 3rd Infantry, the 4th Infantry, the 5th Infantry, the 6th Infantry, the 7th Infantry, and the 8th Infantry.
The battalion served as artillery for much of the war, and despite later being formally designated as infantry, it still retained artillery pieces throughout the conflict. In many ways the battalion acted as the sole Mexican counter-balance to the U.S. innovation of "flying artillery".
Many theories exist on motivations for desertion, including cultural alienation, mistreatment of immigrant conscripts by other nativist soldiers and senior officers, not being allowed to attend Sunday Mass or to practice their religion freely, the incentive of land in Mexico (starting at 320 acres) and witnessing the conduct of U.S. troops following battle victories.
One school of thought is that based on the evidence of the number of Irish Catholics in the Battalion, the letters of Riley, and the field entries of senior officers the primary motivations seem to have been shared religion and sympathy for the Mexican cause, likely based on similarities between the situations in Mexico and Ireland. Another is that the members of the Saint Patrick's Battalion were malcontents who weren't happy with their lot, switching sides for higher wages and land grants. Relevant historical considerations include Irish expatriates' long tradition of serving in Catholic countries' military forces dating back to the Flight of the Wild Geese in the 17th century, and more recent history detailing of the role played by Irish soldiers in South American wars of independence.
John Riley himself mentioned in a brief missive, regarding the appearance of the flag:
According to an American journalist covering the war with Mexico: Two other eye-witness accounts of the flag exist, both from American soldiers. The first describes it as: The second only mentioning: Another radically different version of the flag, as described in this Mexican source: Whatever the case, a reproduction military flag was created by the Clifden and Connemara Heritage Group in 1997, and another the following year for the MGM film One Man's Hero. The film was a romanticised version of the San Patricios' history. A third version embodying the description of the San Luis Potosí flag was produced by the Irish Society of Chicago and hangs in Chicago's Union League Club.
The Saint Patrick's Battalion first fought as a recognised Mexican unit in the Battle of Monterrey on 21 September 1846 as an artillery battery. Popularly they were called Los Colorados by the Mexicans because of their ruddy, sun burnt complexions and red hair color. They were commanded by John Riley, an Irish artilleryman and veteran Non-commissioned officer of the British Army, who possibly arrived in Canada in 1843 whilst serving in the British Army (the assertion that he served as a Sergent in the 66th (Berkshire) Regiment of Foot, is known to be a fabrication) going on to join the U.S. Army in Michigan in September 1845. Considered a competent soldier, Riley was to be promoted to the rank of lieutenant shortly before his decision to desert. This did not affect his decision, and he deserted in Matamoros in April 1846. Upon meeting Mexican forces he was initially given the Officers rank of Lieutenant by General Pedro de Ampudia.
At the battle of Monterrey, the San Patricios proved their artillery skills by mowing down many American soldiers, and they are credited with defeating two to three separate assaults into the heart of the city. Among their targets were companies led by such officers as Braxton Bragg, many of whose soldiers would end up in their own ranks later in the war. Their tenacity, however, did not affect the Mexican commanders decision to capitulate and abandon the position.
Following the engagement at Monterrey, the San Patricios grew in number, by some estimates reaching an enlistment of over 700 men. The U.S. army's conduct at the previous battle, which had included firing on civilians taking refuge in Catholic churches, resulted in more desertions from the U.S. army. Notices were sent out encouraging foreign and particularly Catholic U.S. troops to desert the army and nation that had no respect for them or their religion. Not all the new recruits were deserters, but also European Catholics already residing in Mexico. Forces re-assembled at San Luis Potosí and they had their distinct Green silk flag embroidered there.
As an infantry unit, the San Patricios continued to serve with distinction. Knowing that they were likely to face the death penalty if captured, the San Patricios are known to have threatened wavering Mexican troops with death by "friendly fire" at the Battle of Cerro Gordo, if they retreated. When the San Patricios were too heavily engaged to carry out their threat, the Mexican troops broke and ran, leaving the San Patricios as they fought U.S. troops in hand to hand combat.
The Battle of Churubusco (20 August 1847) took place shortly after the debacle at Cerro Gordo being only two days later. General Santa Anna gave a verbal order to "preserve the point at all risk". The San Patricio Companies initially met the attackers outside the walls of the convent at a tête-de-pont, which was about 500 yards from a fortified convent. A battery of three to five heavy cannons were used from this position to hold off the American advance along with support from Los Independencia Batallón and Los Bravos Batallón. Several U.S. charges towards the bridgehead were thrown off, with the San Patricio companies serving as an example to the supporting battalions. Unlike the San Patricios who mostly consisted of hardened veterans(many having served in the militaries of the United Kingdom and assorted German States, as well as the militaries of various other nations before the Mexican-American war) the supporting Mexican battalions were simply militia(the term National Guard is also used) who up until this point had been untested by battle.
A lack of ammunition started to cause the Mexican soldiers in the trenches between the bridgehead and the convent to disband, with it having been exhasted and leaving them with no effective means of fighting back. To make matters worse Santa Anna had ordered half of these soldiers to a different part of the battle-field. When the requested ammunition wagon finally arrived, the 9 ½ drachm cartridges were compatible with none but the San Patricio Companies "Brown Bess" rifles, with them only making up a faction of the defending forces. Further hampering Mexican efforts, a stray spark from an artillery piece firing grape shot at the on-coming U.S. troops caused the just arrived ammunition to explode and set fire to several men, including Captain O'Leary and General Anaya. A withdrawal behind the walls of the convento de Churubusco was called when due to these factors the threat of being outflanked proved too great.
The San Patricios that had defected from the U.S. army used this battle as a chance to settle old scores, and it was noted that "The large number of officers killed in the affair was...ascribed to them, as for the gratification of their revenge they aimed at no other objects during the engagement". Though hopelessly outnumbered and underequipped the defenders repelled the attacking U.S. forces from the parapets with heavy losses until their ammunition ran out, and a Mexican officer raised the white flag of surrender. Officer Patrick Dalton of the San Patricios tore the white flag down, prompting General Pedro Anaya to order his men to fight on with their bare hands if necessary. American Private Ballantine reported that when the Mexicans attempted to raise the white flag a further two more times, members of the San Patricios shot and killed them. Finally after brutal close-quarters fighting with bayonets and sabers through the halls and rooms inside the convent, U.S. army Captain James M. Smith suggested a surrender after raising his white handkerchief. Following the U.S. victory the Americans "ventilat[ed] their vocabulary of Saxon expletives, not very "courteously," on Riley and his beautiful disciples of St. Patrick".
General Anaya states in his written battle report that 35 San Patricios were killed, 85 taken prisoner (including a wounded John Riley, Captain O'Leary, and Anaya himself having been captured) and about 85 more escaped with retreating Mexican forces. They were reformed before the Battle of Mexico City some two weeks later and remained stationed at Querétaro, made up of the uncaptured survivors of the battle of Churubusco and a roughly equal number of fresh deserters from the U.S. army. Following the war the battalion never regained its former numbers and was officially mustered out of Mexican military service in 1848, either because of members alleged involvement in an abortive military coup or budget cuts.
Two separate courts-martial were held, one at Tacubaya on 23 August, and another at San Ángel on 26 August. At neither of these trials were the men represented by lawyers nor were transcripts taken of the proceedings. Neither was required by law. This lack of formal legal advice could account for the fact that several of the men claimed that drunkenness had led them to desert (a very common defense in military trials at the time that sometimes led to lighter sentences), and others described how they were forced to join the Mexican army in some form or another. The vast majority of the San Patricios either offered no defense or their defenses were not recorded. In any case, the law required death as one punishment for the crime of desertion during a time of war.
The fate that awaited most of the captured San Patricios was death by hanging, thirty from the Tacubaya trial and eighteen from San Ángel. The rationale for this was that they had entered Mexican military service following the declaration of war. This was in violation of the Articles of War for the time which clearly stipulated that the penalty for desertion and/or defecting to the enemy during a time of war was death by firing squad, regardless of the circumstances. Hanging was reserved only for spies (without uniform) and for "atrocities against civilians.", neither of which activities any members of the Saint Patrick's Battalion were charged with. In fact more than 9,000 U.S. soldiers deserted during the Mexican-American War, and only the San Patricios were punished in this way.
Those who had left military service before the official declaration of war on Mexico (Riley among them) on the other hand were sentenced 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.
Bring the damned son of a bitch out! My order was to hang 30 and by God I'll do it!
After four and a half hours the U.S. flag appeared on the flagpole at 9.30 am, the Mexican flag having been taken with cadet Juan Escutia to his death after leaping with it from Chapultepec Castle to deny the Americans the honor of capturing it. In a final act of defiance the men about to be hanged cheered the flag, as one onlooker remarked; "Hands tied, feet tied, their voices still free". At Harney's signal, the carts holding the tied and noosed men pulled away. Harney’s further violations of the Articles of War requiring prompt execution did not result in charges being brought against him. He was subsequently promoted to Brigadier General, a post which he held while the US Army occupied Mexico City. The Mexican government described the manner of the hangings as “a cruel death or horrible torments, improper in a civilised age, and [ironic] for a people who aspire to the title of illustrious and humane”, and by a writer covering the war as "a refinement of cruelty and...fiendish".
The Batallón de San Patricio is memorialised on two separate days; the first on 12 September, the generally accepted anniversary of the executions, and the other on Saint Patrick's Day. The San Patricios are also remembered with many schools, churches and other landmarks taking their name. The street in front of the Irish School, in suburban Monterrey, is named Batallón de San Patricio ("Battalion of Saint Patrick"). Down south, the street in front of the Santa María de Churubusco convent in Mexico City was named Mártires Irlandeses ("the Irish martyrs"). The battalion's name is written in gold letters in the chamber of Mexico's House of Representatives.
In the United States, remembrance of the battalion has historically been different. It was the U.S. army's policy to deny the existence of the Saint Patrick's battalion as a cover-up, only ended in 1915 with an inquiry initiated by U.S. congressmen William Henry Coleman and Frank L. Greene. This resulted in the U.S. army admitting that it been dishonest regarding their knowledge on the matter, and with a congressional ruling ordering the army to turn over its records on the battalion to the national archives. Irish Americans have in the past sought to distance themselves from the battalion.
In 1997, President Ernesto Zedillo commemorated the 150th anniversary of the execution of the San Patricios at a ceremony in Mexico City's San Jacinto Plaza, where the first twenty hangings were staged. Both the Republic of Ireland and Mexico jointly issued commemorative postage stamps to mark this anniversary.
In 2004, at an official ceremony attended by numerous international dignitaries, including directors Lance and Jason Hool, as well as several actors from the film One Man's Hero, a statue was donated by the Mexican government to the Irish government in perpetual thanks for the bravery, honor and sacrifice of the St. Patrick's Battalion. The statue also stands at Clifden's town center.
In recent times the impact of the Saint Patrick's Battalion's legacy has been diverse; it has served as the inspiration for the title of soccer team club Deportivo Chivas USA's supporters association, was evoked in a Saint Patrick's Day message from Subcomandante Marcos of the Zapatista Army of National Liberation, and, due to its cosmopolitan nature, has been remembered as a symbol of international solidarity with Mexico. In honor of John Riley, the Mexican flag flies on September 12 in the town center of Riley's native Clifden, County Galway. There is also a monument dedicated to the battalion there.
b. Variably spelled in English as Jon Reily, Riely, Reilly, O'Reily and O'Reilly. His name is given as Juan Reyle, Reley, Reely and Reiley in Mexican army documents written in Spainsh. Regardless of other variant spellings, the name was Seán O' Raghailligh in the original Irish Gaelic.