Confronted by enormous numbers of Irish-Catholic immigrants in the 1840s, American nativism reared its ugly head. All the world knows, wrote historian Thomas Gallagher, that Yankee hates Paddy. And so it seemed to those who had survived the perilous journey to America only to be labeled inferior by demagogic politicians and feared by Anglo-American workmen. Victims of prejudice in the New World, it should not be considered strange that they would shortly find themselves becoming sympathetic to the Mexicans. Here was another Catholic people being invaded by Protestant foreigners. According to a contemporary account, On reaching Mexico they discovered they had been hired by heretics to slaughter brethren of their own church. On top of this they were confronted with the hatred of their fellow soldiers.The movie about the San Patricios with Tom Berenger is worth seeing, if just for the portrayal of brave young men whose Catholic faith came before all else. It is a story that I had little or no knowledge of but will now not ever forget.
The intense prejudice of many of the American soldiers, especially the volunteers, has been commented upon by at least one careful historian. According to K. Jack Bauer, author of The Mexican War, 1846–48:
The majority of American soldiers were products of a militantly Protestant culture that still viewed Catholicism as a misdirected and misbegotten religion. Although the regulars included a significant number of Catholic enlisted men, the volunteers did not. This strengthened the tendency to ignore the rights and privileges of the Church in a Catholic country as well as increase the harassing of that Church. Some of the volunteers’ acts, like the stabling of horses in the Shrine of San Francisco in Monterrey, so upset the Mexicans that they still mention it in modern works...While in the early Republic there was some tolerance of Catholic minorities, this was to change quickly with the increase in immigration of Irish Catholics during the 1830s and 1840s, reaching its crest during the years of the Irish famine as poor, rural Catholics flooded into the American towns and cities. Anti-Catholic riots broke out in Philadelphia in 1844, and when they were over, the Irish ghetto lay in ruins, hundreds of homeless Irish roamed the streets, and two Catholic churches were burned to the ground.
Since solidarity in the face of commonly perceived oppression is a universal characteristic of any ethnic or religious group, it is hardly surprising that Irish Catholics would find unity among themselves in the military service. As the war progressed and they witnessed more depredations against their coreligionists in Mexico, it is understandable that some Irishmen felt they had more in common with the Mexicans than the invading Americans. The destruction of Catholic churches in Mexico by the invading U.S. army and other depredations by Protestant volunteers had also been well-documented by both sides... Many Irishmen were quick to see that higher loyalties should prevail, and they joined the Mexican side. They simply had more in common with the Mexicans than with the invaders.