The Executive Order appeared to be a direct response to the bombing of Pearl Harbor.
None of these individuals were charged with a crime. There was no evidence that any of them had done anything wrong or was spying for Japan.
There was a long history in the United States of denying citizenship rights to persons of Japanese ancestry.
The FBI had been monitoring Japanese American activity for several years before the war broke out, and right after the bombing of Pearl Harbor, swept into Japanese American communities and arrested possible subversives. FBI head J. Edgar Hoover told President Roosevelt that the West Coast was secure, and he recommended that the Japanese American communities be watched. He did not recommend mass arrests.
U.S. Attorney General Francis Biddle warned President Roosevelt that the forced removal of American citizens was unconstitutional. Although we were also at war with Germany and Italy, there was no mass detention of German Americans or Italian Americans.
Anti-Japanese sentiment on the West Coast increased dramatically once Pearl Harbor was bombed.
There were a number of anti-Japanese organizations in California that had long opposed citizens of Japanese ancestry and Japanese aliens. Did the government think they were protecting Japanese Americans by moving them out of harms way until the war was over?
One argument used by the California Attorney General Earl Warren was that Japanese American loyalty to the United States and the lack of sabotage only proved that Japanese Americans would soon become disloyal.