Thomas J. Craughwell is the author of The Greatest Brigade: How the Irish Brigade Cleared the Way to Victory in the Civil War (Fair Winds, 2011) and Stealing Lincoln's Body (Harvard University Press, 2007).

Civil war historians and enthusiasts will argue over the greatest Confederate general, or whether Mary Todd Lincoln​ was certifiable or just a bit quirky. But when it comes to naming the greatest Union fighting outfit, most will agree it was the Irish Brigade. Comprised of the 63rd, 69th, and 88th New York Infantry Regiments, and eventually the 116th Pennsylvania and the 28th Massachusetts, the Irish Brigade fought in every major battle of the eastern theater of the war, from Bull Run to Appomattox. And they lost more casualties than any other brigade—approximately 4,000. Their courage in battle, sometimes bordering on recklessness, won them the admiration of their Southern foes and made Abraham Lincoln​ express the wish that he had two or three more Irish Brigades.

Yet in the months leading up to the Civil War, it was an open question whether Irish immigrants in the North would fight for the Union. Everyone from parish priests to the publishers of Irish newspapers was urging the Irish to sit out the war. And they had their reasons. America had not been terribly welcoming to Irish Catholic immigrants. Beginning in the 1830s, when immigration from Ireland became pretty steady, a considerable portion of native-born Protestant Americans came to regard the Irish as a threat. The anti-Irish faction became even more alarmed when, between 1847 and 1851, approximately 848,000 Irish arrived in New York City—163,000 of them in 1851 alone. Samuel F. B. Morse, inventor of the telegraph, joined with prominent New Yorkers to found the Native American Democratic Association, a political organization dedicated to restricting immigration from Ireland, requiring a 21-year waiting period before immigrants could become American citizens, and barring from political office anyone who “recognizes any allegiance or obligation of any description to any foreign prince, potentate or power”—in other words, no political office for anyone who recognized the spiritual authority of the pope.

By the 1850s, the Nativists, or Know-Nothings, as they were called (because members were instructed that when asked about the party’s secret activities they should reply, “I know nothing”), were a well-organized political movement. Their candidates were elected mayors of Boston, Philadelphia, New York, Baltimore, Chicago, New Orleans, and Washington, D.C. They dominated state politics in all the New England states, as well as in Pennsylvania, Indiana, and California. But they did not limit themselves to politics—in cities and towns from Bath, Maine, to Galveston, Texas, Nativist mobs destroyed Catholic churches and institutions and burned down the homes of American Catholics.

A few weeks before the first shot was fired on Fort Sumter, the editors of the New York Times​, the leading Republican newspaper in the city at the time, published an editorial linking Catholicism—“popery,” they called it—with slavery as two institutions “incompatible with the spirit of the age, and liberty and civilization.” The editors went on to say that they looked forward to the “speedy destruction” of both.

Anti-Irish, anti-Catholic bigotry was not the only issue. Most of the Irish, especially the Famine Irish, had never done any kind of work but tenant farming. Yet when they arrived in America, the overwhelming majority settled in cities. With no marketable skills, the Irish supported themselves by doing the heavy, dangerous, menial jobs few native-born Americans wanted. The men and boys dug the canals, laid the railroad tracks, and loaded and unloaded cargo on the docks; the women and girls worked as servants or in factories and mills. At the time, the Irish were almost the lowest-paid workers in the United States; the only group that was paid less were free blacks. And that is what worried the Irish: if the Union won the war and Lincoln freed the slaves, they would be competing for jobs with 4 million newly freed men and women who would work for even lower wages than they did...