May 19, 1921 – The United States Congress passes the Emergency Quota Act establishing national quotas on immigration

The Emergency Quota Act, also known as the Emergency Immigration Act of 1921, the Immigration Restriction Act of 1921, the Per Centum Law, and the Johnson Quota Act, was formulated mainly in response to the large influx of Southern and Eastern Europeans and successfully restricted their immigration as well as that of other "undesirables" to the United States. Although intended as temporary legislation, it proved, in the long run, the most important turning-point in American immigration policy because it added two new features to American immigration law: numerical limits on immigration and the use of a quota system for establishing those limits, which came to be known as the National Origins Formula.  ​The use of the National Origins Formula continued until it was replaced by the Immigration and Nationality Act of 1965, which introduced a system of preferences, based on immigrants' skills and family relationships with US citizens or US residents.