Commemorative plaque placed at the San Jacinto Plaza in the district of San Ángel
, Mexico City
in 1959: "In memory of the Irish soldiers of the heroic St. Patrick's Battalion, martyrs who gave their lives to the Mexican cause in the United States' unjust invasion of 1847"
For Mexicans of the generation that fought the Mexican–American War, and generations to come, the San Patricios were heroes who came to their aid in an hour of need. For Americans of that generation, the San Patricios were considered traitors. Successive Mexican presidents have praised the San Patricios; Vicente Fox Quesada stated that "The affinities between Ireland and Mexico go back to the first years of our nation, when our country fought to preserve its national sovereignty... Then, a brave group of Irish soldiers... in a heroic gesture, decided to fight against the foreign ground invasion", and Mexican president Ernesto Zedillo stated "Members of the St. Patrick's Battalion were executed for following their consciences. They were martyred for adhering to the highest ideals ... we honor their memory. In the name of the people of Mexico, I salute today the people of Ireland and express my eternal gratitude".
The great majority of these men were recent immigrants who had arrived at northeastern U.S. ports, part of the Irish diaspora then escaping the Great Irish Famine and extremely poor economic conditions in Ireland, part of the United Kingdom of Great Britain and Ireland at the time. The U.S. Army often recruited Irishmen and other immigrants into military service shortly or sometimes immediately after arrival in America in coffin ships, with promises of salaries and land after the war.
Numerous theories have been proposed as to their motives for desertion, including cultural alienation, mistreatment of immigrant soldiers by nativist soldiers and senior officers, brutal military discipline and dislike of service in the U.S. military, being forced to attend Protestant church services and being unable to practice their Catholic religion freely as well as religious ideological convictions, the incentive of higher wages and land grants starting at 320 acres (1.3 km2) offered by Mexico, and viewing the U.S. invasion of Mexico as unjust.
It is believed primary motivations were shared religion with the Mexicans and sympathy for the Mexican cause based on similarities between the situations in Mexico and Ireland. This hypothesis is based on evidence of the number of Irish Catholics in the Battalion, the letters of John Riley, and the field entries of senior officers. Another hypothesis is that the members of the Saint Patrick's Battalion had been unhappy with their treatment in the U.S. Army; this was the conviction of George Ballentine, an Englishman who served in the American army. Ballentine stated that while "there was a portion of truth" in the view—commonly assigned by officers—that the deserters joined the Mexican army due to their Catholicism, "I have good reason to believe, in fact in some cases I know, that harsh and unjust treatment by their officers operated far more strongly than any other consideration to produce the deplorable result [desertion]," describing how he found the punishments used for "trivial offensives" to be "revolting and disgusting". Another theory some historians hold is that the soldiers were attracted by the incentives offered by the Mexican government: safe passage throughout Mexico for deserters, generous land grants, and the offer of potential military commissions. For poor people coming from famine conditions, economics was often an important incentive.
Mexican author José Raúl Conseco noted that many Irish lived in northern Texas, and were forced to move south due to regional insecurity. Mainly Irish settlers from San Patricio, Texas had previously sided with Mexican forces against Texan rebels at the Battle of Lipantitlán in the Texan Revolution.
Irish expatriates had a long tradition of serving in military forces of Catholic countries, for instance, serving with Spain and later France in groups of young men who had left Ireland during what would become known as the Flight of the Wild Geese in the 17th century. In addition, many Irish fought as soldiers in South American wars of independence.[c]