If Europeans are desirable immigrants, why did the US limit immigration?

Why did the US government severely limit immigration from Eastern and Southern Europe in the 1920s? Aren't white immigrants good for America? Then why did Congress bar the door and restrict the numbers permitted to enter? Before this time, immigrants did not need a visa to enter. From 1920s on, people needed... show more Why did the US government severely limit immigration from Eastern and Southern Europe in the 1920s?
Aren't white immigrants good for America? Then why did Congress bar the door and restrict the numbers permitted to enter?
Before this time, immigrants did not need a visa to enter. From 1920s on, people needed paperwork to qualify as "legal immigrants."
http://historymatters.gmu.edu/d/5078
http://en.wikipedia.org/wiki/Immigration_Act_of_1924
http://history.state.gov/milestones/1921-1936/ImmigrationAct
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