While there were attempts to regulate immigration to the United States as early as 1903, the real impact on immigration came in the 1920s and 1930s. In the early 1900s, some laws were passed restricting the immigration of radicals to our country. However, this really had a minimal impact on immigration trends. In fact, immigration increased between 1901-1910. It was after World War I that immigration really dropped.
After World War I, there was a fear that the Communists were coming to take over our country. There were many strikes that were tied to radical groups. There were anarchists who lived in our country who advocated the overthrowing of our government. Thus, Congress passed two laws that restricted immigration to our country from South and East Europe. It was from these regions in Europe that the radicals were coming. The Emergency Quota Act and the National Origins Act were two laws of the 1920s that significantly restricted immigration to the United States, especially from South and East Europe. The Great Depression also helped to significantly slow immigration to the United States.
As a result of the Great Depression and the passage of several laws, especially those in the 1920s, immigration to the United States slowed between 1910-1940.