In the early days of World War II, orders were issued by the Pacific coast military commander (lieutenant general John L. DeWitt, head of the Western Defense Command), the Attorney General of California (Earl Warren), and eventually President Franklin D. Roosevelt, in Executive Order 9066, for the extrajudicial detention of all persons of Japanese ancestry, whether citizens of resident aliens. Under Presidential proclamation and emergency legislation, virtually all persons of Japanese ancestry, who resided on the Pacific Coast of the U.S. mainland, were sent to detention camps.
Actual Japanese threats in WWII
At the same time, in the Hawaiian Islands, which had actually been attacked, U.S. Army recruiters were overwhelmed with men demanding to enlist, and the 442nd Regimental Combat Team was formed. It was sent to fight in Europe, not in the Pacific, and, for a unit of its size, had the highest proportion of awards for valor of any U.S. Army combat formation. There was some discrimination at that time; some previously denied Medal of Honor recommendations, such as to Sen. Daniel Inouye (D-HI), eventually received it.
Were there cases of Japanese espionage or sabotage? Very few, usually by persons in contact with Japanese diplomats, or detected through counterintelligence or communications intelligence.
Other American soldiers of Japanese ancestry served, with distinction, as translators and intelligence analysts, as well as occasionally in combat, in individual cases in the Pacific. They generally had not lived in the designated Pacific "Western Defense Zone".
On July 2, 1948, Congress passed the Japanese American Evacuation Claims Act to provide compensation to Japanese properties damaged during the "relocation".
In 1980 the Congress opened an investigation to the internment program and a report titled "Personal Justice Denied" was written. The report condemned the "relocation" and the Korematsu court decision.
In 2001, the PBS broadcast a Eric Paul Fournier film Of Civil Wrongs and Rights in memory of the Japanese internment and the Korematsu litigation.  Warren, in his autobiography, said
I have since deeply regretted the removal order and my own testimony advocating it, because it was not in keeping with our American concept of freedom and the rights of citizens. Whenever I thought of the innocent little children who were torn from home, school friends and congenial surroundings, I was conscience-stricken.
- Timothy P. Fong et al., ed., February 19, 1976: President Gerald Ford formally rescinds Executive Order 9066., Robert T. Matsui Legacy Project, California State University, Sacramento
- Timothy P. Fong et al., ed., July 2, 1948: Congress passes the Japanese American Evacuation Claims Act., Robert T. Matsui Legacy Project, California State University, Sacramento
- Commission on Wartime Relocation and Internment of Civilians (December 1982), Personal Justice Denied
- Of Civil Wrongs and Rights: The Fred Korematsu Story, Public Broadcasting Service, December 2001
- Earl Warren, The Memoirs of Chief Justice Earl Warren, Madison Books, 2001, cited in