The document provides background information on immigration to the United States in the 19th and early 20th centuries, when millions of Europeans immigrated seeking opportunity. It then discusses five factors that led Americans to change their attitudes towards immigrants in the 1920s: isolationism, fear of revolution, prejudice and racism, economic and social fears, and the effects of World War I. Each factor is explained with examples of how it contributed to more negative views of immigrants. The document aims to help students understand and analyze the reasons for shifting immigration policies in that era.