Between 1900 and 1915, more than 15 million immigrants arrived in the United States. That was about equal to the number of immigrants who had arrived in the previous 40 years combined. In 1910, three-fourths of New York City's population were either immigrants or first generation Americans.

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Not only were the numbers of immigrants swelling, the countries from which they came had changed dramatically as well. Unlike earlier immigrants, the majority of the newcomers after 1900 came from non-English speaking European countries. The principal source of immigrants was  Italy, Poland, and Russia, countries quite different in culture and language from the United States. The so-called "new immigrants" had difficulty adjusting to life here.

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At the same time, the United States had difficulty absorbing the immigrants. Most of the immigrants chose to settle in American cities, where jobs were located. As a result, the cities became ever more crowded. In addition, city services often failed to keep up with the flow of newcomers. Most of the immigrants did find jobs, although they often worked in jobs that most native-born Americans would not take. Over time, however, the immigrants succeeded in bettering their condition.

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