If you have an immigrant ancestor who did not become a U.S. citizen prior to 1940, then you may find a search for an alien registration record worth your time. The Alien Registration Act of 1940 (also known as the Smith Act) required all non-citizens 14 years of age or older to register at their local Post Office or INS office (now the USCIS, or U.S. Citizenship & Immigration Service) beginning in June 1940. A great deal of information of interest to family historians was collected, including full names, place and date of birth, date and port of arrival to the U.S., whether the alien ever applied for citizenship; the names of parents, spouse and children in the U.S., and even a fingerprint! More than 4 million alien residents were registered during the first year of the program.
Alien Registration records for the WWII period of 1940-1944 were microfilmed by the INS (USCIS) for their internal use, but copies can be obtained through an online request with the USCIS Genealogy Service. If the immigrant's date of birth is less than 100 years prior to the date of this request, you must provide documentary evidence showing the immigrant is deceased. Most record requests are fulfilled within a maximum of 90 days.
Alien registration also occurred in the United States during 1917 and 1918, following the onset of World War I. Unfortunately, many of these records have been destroyed through the years, but a number of scattered records are held in state archives and other repositories across the United States. Some WWI alien registration records can be found indexed online, including those for Kansas; Phoenix, Arizona; and St. Paul, Minnesota. FamilySearch has an index and images for San Francisco (California) World War I Enemy Alien Registration Affidavits. Other alien registration records are available in offline repositories, such as the 1918 Minnesota Alien Registration records at the Iron Range Research Center in Chisholm, MN.
The United States was not the only country to undertake registration of its alien residents during wartime. Alien registration records of non-British subjects in Australia was required between 1916 and 1926, and again between 1939 and 1971. Many of these records are available in the online Record Search feature of the National Archives of Australia. A small sampling (600 out of tens of thousands of aliens resident in London since 1914 can also be viewed online from the UK National Archives.