During the World War II, the US authorities decided to relocate and incarcerate the Japanese citizens into concentration camps. The majority of the Japanese diaspora lived on and around the West Coast, so this is where it all happened. It is estimated that between 110,000 and 120,000 people that had Japanese ancestry were taken into these camps. The reason was that the US authorities feared that these people will spy for the Japanese, and that they will be their ears and eyes on their soil, though there was not a single proof for such a thing.
The Korematsu vs US was a court case in the US where Korematsu was trying to convince the US authorities that the people of Japanese ancestry do not pose any threat to the national security, and that they should be released as they are kept illegally as prisoners.