I've been doing some light (heh) reading on the limits of presidential power during war, and Korematsu of course came up. I read that the internment order was based on claims like signal lights in California or something along those lines. I read that the report was disavowed, but was there even credible reason at the time internment was adopted to believe that the intelligence was based on sound information? Or was the US duped?
While much of the reasoning was just the conclusion of about a century of West Coast racial tensions, and the Navy seeking to just have its way while trying to stave off both civilian and military panic in early months of 1942.
But beyond the tension, the Niihau Incident also could not be ignored, and played on lots of the prejudices and fears of decision makers. The fact that not just Issei but Nisei as well aided in the attempt to escape of a downed IJN Aviator was troubling. The fact that they were willing to turn on their native Hawaiian neighbors and aid in his activities, while knowing of the attack was seen as a clear sign that otherwise outwardly loyal persons were just waiting for the means and opportunity to act out. Of course the major population of Japanese on the islands, and the already huge military population and general restrictions accomplished many of the goals for security without disrupting the necessary functions of the islands. Such considerations and dynamics that simply did not exist on the Mainland.