Between 1820, when the federal government began to count arrivals, and 1860, almost 5 million European immigrants arrived in the United States. Although the number declined during the first years of the Civil War, it began building again by 1863. During the course of the 1860s, about 2 million immigrants came, with another 2.7 million entering in the 1870s. Never before had the country needed to absorb so large a number of newcomers in so short a time. The range of countries from which immigration took place also increased, with fewer people coming from Britain and more from Ireland, Germany, and the Scandinavian countries. In addition, under the Treaty of Guadalupe Hidalgo, the United States gained control over large areas of what had been Mexico. Although they were not, strictly speaking, immigrants because they had already been living in what became U.S. territory, under the treaty Spanish speakers found themselves under American rule. Their numbers were relatively small, however. The 1850 census, for example, counted only 13,000 Mexican nationals living in the United States.

Most of the new immigrants came as free workers, since indentured service had declined in importance as a mechanism of immigration. Concepts about the new immigrants largely followed notions as to their assimilation. Observers such as Herman Melville shared the optimism of the Pennsylvania model: “On this Western Hemisphere all tribes and people are forming into one federate whole; and there is a future which shall see the estranged children of Adam restored as to the old hearthstone in Eden” (Melville 1983: 185). That is not to say that even supporters of immigration were not concerned about the assimilation of newcomers – certainly, concerns about English language acquisition persisted. However, those who believed in the Pennsylvania model generally trusted the absorptive capacity of the republic.