Between 1820 and 1840, an estimated 700,000 immigrants arrived in the United States, mainly from the British Isles and German-speaking areas of continental Europe. During the 1840s, this substantial flow suddenly became a flood. The largest single source of the new mass immigration was Ireland, but Germany was not far behind. Smaller contingents came from Switzerland, Norway, Sweden, and the Netherlands.
The massive transatlantic movement had many causes; some people were “pushed” out of their homes while others were “pulled” toward America. The great push factor that caused 1.5 million Irish to forsake the Emerald Isle between 1845 and 1854 was the great potato blight, which brought famine to a population that subsisted on this single crop. Escape to America was made possible by the low fares then prevailing on sailing ships bound from England to North America. Ships involved in the timber trade carried their bulky cargoes from Boston or Halifax to Liverpool; as an alternative to returning to America partly in ballast, they packed Irish immigrants into their holds. The squalor and misery in these steerage accommodations were almost beyond belief.
Robert A. Divine, et al., The American Story 3rd ed., vol. 1 (New York: Pearson Education, Inc., 2007), 348.
The tidal wave of immigrants that broke over American in the decade from 1845 to 1855 produced a nasty backlash among Protestant Americans, who believed they were about to drown in a sea of Roman Catholics from Ireland and Germany…The new arrivals encountered economic prejudice, ethnic hostility, and religious antagonism. Because some of them displayed a taste for whiskey and beer, they also drew the wrath of the temperance movement. When the immigrants entered American politics, they largely became Democrats because they perceived that party as more tolerant of newcomers than were the Whigs.
But even so, they met sharp political opposition. In the early 1850s, nativists (individuals who were anti-immigrant) began to organize, first into secret fraternal societies such as the Order of the Star-Spangled Banner, and then into a political party.
Recruits swore never to vote for either foreign-born or Roman Catholic candidates and not to reveal any information about the organization. When questioned, they said: “I know nothing.” Officially, they were the American Party, but most Americans called them Know-Nothings.
James L. Roark, et al., eds., The American Promise: A History of the United States, 2nd ed., vol. 1 (Boston: Bedford/St. Martin’s, 2002), 466-467.