The war between the United States and Mexico had two causes: “Manifest Destiny,” the desire of the U.S. to expand its territories under the belief that Americans had a God-given right and duty to “civilize the whole continent,” and the Texas War of Independence.

In 1844, with the election of President James K. Polk, an avowed expansionist, the United States embarked on a course to acquire the lands west to the Pacific Ocean. Polk had authorized his envoy John Slidell to offer $5 million for Texas, $5 million for New Mexico and up to $25 million for California, but the offers were refused by Mexico. Slidell’s formal instructions were to negotiate, adjust boundaries and other causes of differences under fair and equitable principles. To the Mexicans this meant, “Accept our terms or face the consequences.”

Many Mexicans still refused to accept the annexation of Texas to the U.S. in 1836 under the Treaty of Velasco which was signed by General Santa Anna. Captured in the battle of San Jacinto, Santa Anna was a prisoner of the Texans at the time of the signing.

After many savage border fights, Texas decided to join the United States on July 4, 1845. Mexico was not happy with its breakaway province, which now claimed the border at the Rio Grande River. A major international issue and a tense standoff ensued.

On April 2, 1846, a clash occurred between Mexican and American troops on soil that was claimed by both.

President Polk, in his declaration of war, stated that, “American blood had been shed on American soil.” In truth, the war had been planned even before the news of the Mexican attack on the American patrol had been received.

Despite early popularity, the war had its opponents. There was great opposition to the war by the Whig Party and some members of the U.S. Army. Ulysses S. Grant, later General Grant, wrote in his memoirs, “I was bitterly opposed to the Annexation of Texas measure, and to this day regard the war that resulted, as one of the most unjust ever waged by a stronger against a weaker nation. It was an instance of a republic following the bad example of European monarchies, in not considering justice in their desire to acquire additional territory.”

Former President John Quincy Adams described the war as a Southern expedition to find “bigger pens to cram with slaves,” and Whig congressman from Illinois Abraham Lincoln disputed the location of the skirmish as being American soil and submitted “Spot Resolutions” to Congress.

In order to fight the war, Congress authorized 50,000 troops and $10 million. The offer for volunteers was ten dollars a month with three months advance pay and 160 acres of farmland. Volunteers, including thousands of Irish immigrants newly arrived from famine-stricken Ireland, swarmed the recruiting centers and quotas were filled within weeks.

The San Patricios

Mexico was not to be outdone in terms of recruitment. General Santa Anna encouraged American soldiers to fight on the Mexican side with offers of cash in dollars and 200-acre grants of land. They could retain their rank and pay grade and fight under the leadership of fellow American officers. Estimates as high as 9,000 soldiers deserted from the American army during the Mexican war and many later vanished into the Mexican countryside. The Irish deserters joined together and, under the leadership of Irish-born John Riley, formed the San Patricio Battalion.

The San Patricios created their own military banner with Saint Patrick on one side and a shamrock and the harp of Erin on the other. The reasons given for desertion were bad treatment and poor subsistence they received from non-Catholic members of the American Army. Being Catholics, they also resented the bad treatment given to Mexican civilians, priests and nuns after the war started.

The San Patricios fought in the five major battles against the Americans, which included Matamoros, May 3, 1846, Monterrey, Sept. 21, 1846, Buena Vista, Feb. 22, 1847, Cerro Gordo, April 17, 1847 and Churubusco, August 20, 1847. After the battle of Buena Vista, the San Patricios gained recognition as a Mexican fighting unit to be reckoned with. They gained the grudging respect of the American Army.

Churubusco was the site of one of the bloodiest battles of the Mexican War. The superior tactics and strategy of the American Army, which included Military Academy-trained officers, accurate and fast-loading artillery and the U.S. Army’s 1841 percussion rifle, helped make the assault on the fortress-convent of San Mateo at Churubusco a success. Equipment alone does not win battles; it was the blood and guts of the American soldiers and marines under the command of Major General Winfield Scott that contributed to this victory.

The Castle of Chapultepec, located southwest of Mexico City, was heavily fortified and was a military obstacle that had to be taken prior to entering the city. The castle had been the resort of Aztec princes and since 1833 had served as Mexico’s military academy. The phrase “From the Halls of Montezuma” in the U.S. Marine Corps hymn is based on the battle of Chapultepec. The castle was stormed by a mixed force of American soldiers and marines. About 50 young Mexican cadets refused to leave and — some of them younger than 13 — confronted a bayonet charge. An American correspondent described the youths as “fighting like demons” as some of them fell to their death over the castle wall to the rocks below. They were later immortalized by their countrymen as “Los Ninos Heroicos” — the heroic children.

The story of the San Patricios has been shrouded in legend so the numbers mentioned in this article may have some variation. It was reported that the victory by the Americans led to the capture of the San Patricios, which included Mexican Brevet Major John Riley. It is estimated that as many as 260 San Patricios fought alongside the Mexicans in the battle of Churubusco; 72 were taken prisoners; the rest escaped or were killed in action. General Santa Anna commented, “a few hundred more men like them and we would have won the battle” and praised them for their proficiency and bravery.

Their capture by the Americans led to a verdict of “guilty of desertion” and punishments ranged from two hundred lashes, branding of “D” for desertion, or death by hanging. The penalty of death was not unusual punishment, since most armies imposed a death penalty for desertion during a time of war. Of the fifty sentenced to death, “sixteen were hung by the neck until dead” and two days later, the remaining San Patricios faced the firing squad. The sentences of Mexican Brevet Major John Riley and eleven others were commuted by General Scott because they had deserted before the war with Mexico had been officially declared.

Mexico honored the San Patricios with medals, memorial plaques and annual ceremonies. The U.S. Army regarded them as deserters and traitors, who deserved the punishment they received.

The Irishmen, who had never formed much devotion to America due to the treatment they had received, were unfortunate in choosing the losing side. This did not diminish their bravery, since heroism can surface in the heat of battle on either side of a conflict. The bond of friendship between the Irish and Mexicans still exists, and if you visit Mexico and run across some Mexicans with Irish surnames, they may be descendants of San Patricio Battalion soldiers that escaped from the battle of Churubusco.