excerpted from Wikipedia.com....

By the 1920s, the United States had relatively large populations of many European immigrants spread out over 150 years who had joined the original British descendants majority in America. The foreign born population in the U.S. has never exceeded 15% since before 1675 and has never been a land of immigrant majorities since then. Americans of European ancestry have always been and remain in the majority.

Congress passed the Emergency Quota Act in 1921, followed by the Immigration Act of 1924, which was aimed at further restricting the Southern and Eastern Europeans who had begun to enter the country in large numbers beginning in the 1890s. Most of the European refugees (principally Jews) fleeing the Nazis and World War II were barred from coming to the United States.

In 1924, quotas were set for European immigrants so that no more than 2% of the 1890 immigrant stocks were allowed into America. In addition, Congress passed a literacy act in 1917 to curb the influx of low-skilled immigrants from entering the country.

Immigration restrictions laws passed in the 1920s tried to achieve four goals: reduce drastically the number of unskilled immigrants; favor uniting of families by giving preferences to relatives; keeping the ethnic distribution stable by allocating quotas to various ethnic groups; with no quotas initially set for Mexico and Latin America because of the ongoing Mexican Revolution. In 1900, when the U.S. population was 76 million, there were about 500,000 Hispanics.

The Mexican Revolution of 1911-1929 killed an estimated one million Mexicans and drove at least a million refugees temporarily into the U.S. Many returned in the 1920s or 1930s. The recorded immigration was 219,000 from 1910-1920 and 459,000 from 1920 to 1930. Because of the porous border and the poor or non-existent records from this time period, the real numbers are undoubtedly higher. This recorded number of Mexican immigrants drops to only 23,000 from the decade of 1930 to 1940. Indeed 100,000s returned during the Great Depression either voluntarily or with some U.S. persuasion.