At 9:30 a.m. this day, as the American army raised the Stars & Stripes over Chapultepec Castle during the Mexican-American War, it simultaneously carried out a mass hanging of 30 Irish deserters who had gone over to Santa Anna — the Saint Patrick’s Battalion, or the San Patricios. Irish had been migrating to the United States en masse even well before the Great Famine got rolling in 1845. And for those of that great migration wave who wound up in the service fighting the Mexican-American War, there was a hint of deja vu — an Anglo and Protestant imperial power...