Based on prejudice against the Japanese and on fear that Japanese Americans would provide information or support to Japan in the war.
Explanation:
According to History Matters from George Mason University:
Mostly, though, the Japanese Americans were forced into internment camps as a result of anti-Japanese prejudice and fear. Suspicious of anyone of Japanese heritage, the government restricted the civil liberties of Japanese Americans. In February, 1942, President Roosevelt issued Executive Order 9066, which allowed the Secretary of War to designate certain areas as military zones. FDR's executive order set the stage for the relocation of Japanese-ancestry persons to internment camps. By June of 1942, over 100,000 Japanese Americans were sent to such internment camps.