Immigration developed as a major factor in American politics during the late 19th century, due to the large scale of immigration, the new and more diverse origins of the immigrants, and the resulting social and economic tensions. Immigration contributed to the growth of big-city political "machines," and after World War I, fear of immigrant radicals triggered the first great Red scare in American national politics. Thus, immigration, and the fear of immigration and immigrants, played a major role in American politics, and one that lasted long after the era of mass immigration came to an end.
The sheer volume of immigration to the United States during the decades around the turn of the 20th century was enormous. During the period 1880-1920, 23 million immigrants entered the United States, or on average about half a million each year. The total population of the country in 1900 was only 76 million (roughly a quarter of the current US population). Thus the immigrant flow during these four decades was equal to nearly a third of the population at the midpoint, which gives a sense of the scale and scope of the immigrant impact. The proportion of immigrants in large cities, particularly in the industrial Northeast, was often well over a third of the total population. Moreover, since this was well before the age of large-scale suburban growth, the population within the city limits corresponded more closely to the whole population of what would now be called the metropolitan areas of these cities.
From beginning of the colonies, of course, the future United States had been a society of immigrants and people descended from immigrants. Only the Native American Indians were truly indigenous. However, immigration had previously been a much more gradual progress, and the nation much more rural, so that first-generation immigrants tended to find themselves surrounded largely by the native-born, who had already assimilated to Ame...