Between 1861 and 1940, approximately 275,000 Japanese immigrated to the United States, with the majority arriving between 1898 and 1924, when quotas were adopted that restricted Asian immigration. Assimilation and acceptance proved hard to come by for many of these Japanese immigrants, but following the attack of the Japanese on Pearl Harbor on December 7, 1941 their new lives in America truly fell apart, as the U.S. Congress and the President yielded to pressue to remove individuals of Japanese descent (both issei, first generation of Japanese in the U.S., and nisei, U.S. born, second generation Japanese Americans) from the West Coast states. On February 19, 1942, President Franklin Delano Roosevelt issued Executive Order 9066 authorizing the exclusion - an order implemented by Congress on March 21, 1942 as Public Law 503.
During the next 6 months, approximately 120,000 men, women and children o Japanese ancestry living in California, Oregon, Washington, Arizona, Alaska and Hawaii, were rounded up and transported first to assembly centers near their homes, and then to one of 10 isolated, fenced and guarded relocation centers, commonly referred to as internment camps. Nearly 70,000 of these individuals of Japanese descent were American citizens by birth. Most lost their homes and property - and all lost their freedom and personal liberties - although they had done nothing wrong and no charges were ever made against them.
Records created during the relocation of "potentially dangerous persons" to the relocation centers operated in Washington, Arizona, Alabama, Oregon and California by the War Relocation Authority during WWII can be obtained from the National Archives. They generally include little to help you trace the family back to Japan, but may include parents' names, correspondence and other items of interest, including previous adress, occupation, education, foreign residence, sex and marital status, year of birth, age and birthplace. The Internment Case Files, located in Group 210, Records of the War Relocation Authority, can be obtained by making a Freedom of Information Act request through the National Archives. An index to these records can also be accessed online for free through the National Archives Access to Archives Database. Once you find the record for a specific individual, you can search for records for other family members via the 5-digit "Individual Number" or "File Number." The members of one family usually have the 5-digit number in common, with a following letter denoting the position of the person within the family. Records relating to the compensation to Japanese Americans relocated during WWI for losses of real and personal property can also be searched online in Japanese Claims Act of July 2, 1948 Case Files (Record Group 60).