Why hundreds of American soldiers defected to fight for Mexico in 1846

History · Published 9 October 2026 Swipe more like this →

In short: During the Mexican-American War, a unique unit known as the Saint Patrick's Battalion turned against the United States. Composed largely of Irish immigrants, these soldiers abandoned the American army to defend Catholic Mexico, driven by deep-seated religious prejudice and systemic discrimination they faced within the ranks of their adopted nation.

The Saint Patrick's Battalion, or El Batallón de San Patricio, emerged from a climate of intense anti-Catholic sentiment in the United States. Many Irish immigrants, arriving in droves to escape economic hardship, found themselves marginalized by a Protestant majority that viewed their faith with suspicion. This hostility was not merely social; it manifested in violent episodes like the 1844 Bible Riots in Philadelphia and the burning of a Boston-area convent. Within the U.S. Army, which was comprised of 40 percent immigrants by the start of the war, this bias persisted among the officer class, fostering deep indignation among the rank-and-file soldiers.

John Riley, an Irish immigrant and former artillery instructor for West Point cadets, became the battalion's founder. In the spring of 1846, as U.S. forces gathered along the Rio Grande, Riley crossed the river to offer his services to the Mexican military. His decision reflected a broader disillusionment with the American expansionist agenda, which President James K. Polk pursued under the banner of Manifest Destiny. Even future president Ulysses S. Grant, then a young lieutenant, later characterized the conflict as an unjust war waged by a stronger nation against a weaker one, noting that the political push for annexation was heavily tied to the contentious issue of slavery.

The San Patricios fought with fierce loyalty to the Mexican cause, most notably at the Battle of Churubusco. However, their defiance ended in tragedy. Following their capture, many were subjected to public whippings, branding, and execution. On September 13, 1847, as U.S. artillery bombarded Chapultepec Castle, the final group of captured battalion members was hanged. Today, the legacy of the Saint Patrick's Battalion remains complex; while they were branded as deserters by the United States, they are remembered and celebrated as heroes in Mexico for their ultimate sacrifice in defense of the nation they chose to join.

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