Roosevelt justified the order on the grounds of military necessity, declaring that Japanese Americans were a threat to national security. Anti-Japanese sentiments had been developing in the U.S. long before WWII had even begun.
Roosevelt's infamous solution to this problem was Executive Order #9066, which authorized the internment of between 110,000 and 120,000 Japanese Americans on the pacific coast. Roosevelt justified this authorization on a legal argument that the need to protect the country from espionage outweighed the individual rights of those that were interned.
