Much of the Puritan crusade against Catholicism comes down to crude religious hatred and general xenophobia, with a healthy dose of partisanship on top. We can just write it off as a bigotry of the times and dismiss it, like we would the same sort of ideas today. That would probably suit our stomachs fairly well, and certainly flatter our self-image, but if we can try to understand the paranoia and outright terror that inspired slaveholders to the defense of their institution as making sense in a certain context then we can do the same for anti-immigrant and anti-Catholic hysteria.
Catholics, they imagined, engaged in all manner of political corruption, and sexual depravity on behalf of, and perhaps with direct instructions from, Rome. White Americans applied the same libels to black Americans, the corruption coming during Reconstruction, and the anti-black and anti-Catholic, anti-immigrant movements essentially merged in the decades around the turn of the twentieth century. Over in Europe, generations of anti-Semites applied the same libels to the Jews. At various times they also attached to European Catholics, to various dissenting religious movements, to people from the Mediterranean basin, linguistic minorities, and so forth. We have a real genius for finding ways to excuse mistreatment of one another.
Puritan-minded Protestant Americans, fixed on their vision of a lily-white empire for people with ancestors in the fashionable section of the British Isles, did not need to look far to come on these ideas. The Reformation and consequent religious wars, complete with both propaganda and genuine atrocities, loomed large in their minds. To some degree, the Roman Church represented the ultimate religious horror: an institution devoted to serving Christ but corrupted from within into a Satanic vessel. Even if Protestants behaved terribly toward one another, and they often did, they all had a folk memory of an opulent, corrupt, oppressive church in Rome with designs on them all. But if they needed a reminder to spur their anti-immigrant hysteria, they could get one from the immigrants.
Many of the wave of immigrants sweeping into America in the late 1840s and into the 1850s came as refugees from the European revolutions of 1848. Their Springtime of Nations failed, but they took their nineteenth century liberalism with them across the Atlantic to the one republic that seemed to run on something approaching liberal principles, the United States. The admiration did not go only one way across the ocean, of course. The abolitionists often understood themselves as members of a kind of transatlantic liberal movement that had much in common with efforts to abolish aristocracy and the like in Europe. When the foreign-born population swelled by 84% in the 1850s, it included plenty of European liberals fresh off the boat with new tales of reactionary Catholicism to remind antislavery men of their own religious history.
This all has a strong element of score-settling, of course. The revolutionaries lost their battles to reform Europe. Some of them quit the continent for good, but others intended to go back. Maybe they could go back with an army of Americans. Nineteenth century Americans, including far more conservative men than any abolitionist, adored the idea of teaching bad old Europe from which their ancestors fled how the world should really run. They would be the light of the world. If some American boys wanted to carry a gun alongside that light and go save Europe from itself, nineteenth century America could admire their patriotism and manly vigor.
This went beyond the streets and fringe. When Lajos Kossuth, Louis Kossuth to Anglophones, fled Hungary he found many admirers in the United States. Congress invited him to address a joint session, an honor previously given only to the Marquis de Lafayette. Millard Fillmore entertained Kossuth at the White House. He toured the nation, attending a meeting held in his honor by a failed politician named Abraham Lincoln among many others. Congress fell short of authorizing any kind of foreign adventures on Kossuth’s behalf, but filibusters courted him and he found welcome in the country until he got mixed up in a proposal to take over Haiti and recommended that German Americans vote for Pierce. Those efforts him a partisan and pushed him from the political mainstream.