The St. Patrick's Battalion - known in Spanish as el Batallón de los San Patricios - was a unit comprised primarily of Irish Catholics who had defected from the invading US army during the Mexican-American War and turned their guns against the land grabbing U.S. invasion.
These proud "traitors" worked to show direct solidarity with the Mexican people who faced the brutal effects of the U.S. military invasion. Witnessing atrocities committed against other marginalized people, also mostly Catholics, these Irish and other European migrants found affinity with the Mexican people.
In the 1800's, the U.S. used coercion and force to press many recent migrants into U.S. military service. Many
migrants barely left the boats they arrived in before having recruitment papers forced into their hands and threats made against their families if they failed to enlist. Poor, disparate, hungry, and hopeless, they had little choice but to join the military.
But many of these men made a brave choice. Instead of serving the imperial conquest of the U.S. government, they instead risked all to defend those struggling to resist the invasion.
After the Battle of Churubusco, most members of the battalion were killed or captured: most of those taken prisoner were hanged for treason against the United States, a country that many of them were not even citizens of, and the majority of the others were branded, whipped, and tortured.
To this day, poor and working class folks of European descent have far more in common with Mexican migrants and other working class folk of color than they do with white rich folks. We just have to remember that and choose to bravely fight for the right side.