What was the main cause of the great migration to the united states in late 1800s and early 1900s

The immigrants who came to United States during the 19th and early 20th centuries largely came either for economic opportunities or to escape a negative situation in their home countries. For instance, in the 1840s, Irish immigration peaked during the Irish famine, while Asian immigrants came mostly for jobs on the railroads in the West. Refugees from war and conflict were another regular source of immigrants to the United States.
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