"Korematsu was not excluded from the Military Area because of hostility to him or his race. He was excluded because we are at war with the Japanese Empire, because the properly constituted military authorities feared an invasion of our West Coast and felt constrained to take proper security measures, because they decided that the military urgency of the situation demanded that all citizens of Japanese ancestry be segregated from the West Coast temporarily, and finally, because Congress, reposing its confidence in this time of war in our military leaders-as inevitably it must-determined that they should have the power to do just this. There was evidence of disloyalty on the part of some, the military authorities considered that the need for [323 U.S. 214, 224] action was great, and time was short. We cannot-by availing ourselves of the calm perspective of hindsight-now say that at that time these actions were unjustified."

If you wish to investigate this case in further detail, CompuLEGAL has created Follow Up Questions that identify some of the issues it raises.  You may also want to read the entire Korematsu v. U.S. (1943) & Korematsu v. U.S. (1944) decisions.