WWII started in Europe at the time the United States wanted to be isolated. This meant that the United States did not want anything to do with the war. On December 7, 1914 Japanese attacked the US Pacific Fleet- Pearl Harbor.
After the bombing of Pearl Harbor, President Roosevelt declared war on Japan. So basically the bombing of Pearl Harbor led us to war even though the US wanted to be isolated. The United States had to be prepared because even if they didn't want to be involved they were now in the war.
Once the United States had declared war, Americans were worried that citizens of the Japanese ancestry would act as spies. Therefore what the americans did was they put all people of Japanese ancestry in camps until the war was over. They imprisoned over 127,000 people during the war.