Why was Koremastu and others forced to go to an internment camp

Koremastu and other Japanese Americans were put into internment camps because Japan attacked the United States at Pearl Harbor and this triggered a huge anti-japanese movement in the USA. The internment is considered to have resulted more from racism than from any security risk posed by Japanese Americans. Those who were as little as 1/16 Japanese and orphaned infants with "one drop of Japanese blood" were placed in internment camps.

Simple Answer: Because they were Japanese.