From about 1880 through the mid-1920s, America experienced an immigration boom, “the Great Wave,” during which immigration averaged 600,000 annually. This was the period during which the U.S. industrialized, creating a huge demand for factory workers. The demand was filled primarily by European immigrants; particularly, in its second half, with immigrants from southern and eastern Europe.
In 1924 and 1926, partly in response to pressure from labor unions, Congress put in place the first comprehensive quota systems to limit immigration into the U.S. For the next 40 years, from 1925 to 1965, the United States had a relatively restrictive immigration policy, which allowed 200,000 people into the country annually, on average. Demographers sometimes call this period “the Great Pause,” although at the time, most Americans thought of it as permanent.
In 1965, Congress replaced quotas that had favored immigrants from northern and western Europe, with a new, less racially-biased system, that (in theory) allotted immigration slots as a proportion of total world population. This initiated a new phase in immigration policy, in which non-European immigration predominated; with the subsequent emphasis on “family reunification,” immigration from Mexico and the rest of Latin America came to predominate.
http://www.immigrationeis.org/about-ieis/us-immigration-history