European immigrants in the late nineteenth century brought energy, vigor, and labor power into the still young country. Because they were willing to work hard for very low wages, they were instrumental in creating the industrial rise of the United States that would eventually make it the world's twentieth-century super power.
However, the immigrants increasingly came from places and cultures that pre-existing European immigrants viewed with suspicion and hostility. Before the Civil War, most of the immigrants came to the US from countries such as England, Scotland, Ireland, the German states, and Scandinavia. They were primarily Protestant. (Europeans of other ethnicities and religions had always come but not in great numbers.)
One social change, therefore, was a rise in interest in Colonial and early American roots as a way to distinguish between the "superior" early settlers who were here first and the newcomers. Groups like the Daughters of the American Revolution would create distinctions between those who were "better" and those who did not really belong. Places like Williamsburg were not rebuilt innocently: they were part of a surge in interest in firmly establishing "America" as a northern European Protestant nation, a legacy that lives with us and haunts us to this day.
Thus, along with great increases in wealth, fear grew among the existing populations that these "non-white" and often Catholic and Jewish immigrants from eastern and southern Europe were going to destroy America's perceived purity. This led to both positive and negative changes in society. For example, part of the impetus fueling the woman's suffrage movement was awareness that more of the new, seemingly alien immigrants were male than female. It was understood that giving women the vote would therefore increase proportionally the number of "white" votes. This did not happen until the twentieth century, but part of the groundwork for this positive change was set in the late nineteenth century by increased immigration.
A negative change that took hold in the twentieth century was the racially and religiously motivated restrictions on immigration after World War I that were a response to the deluge of immigrants before the war.
In the end, northern European fears that the United States would be ruined by a deluge of darker skinned, non-Protestant Europeans proved to be false. People from southern and eastern Europe contributed greatly to the vibrance of American culture while becoming enthusiastic citizens.