Lt. General DeWitt was in charge of the military evacuation of the Japanese from the West Coast. He championed the doctrine of "military necessity" rather than unreasoning racism. He claimed, and many others in the government supported this idea, that it was a "military necessity" for the Japanese and Japanese Americans to be evacuated from the military zones (Smith, 122).
Dillon S. Myer's speech "Racism and Reason" reflects the two attitudes that people had: anti-Japanese sentiment was a well-known factor affecting the view people across the United States had towards the internment. The doctrine of "military necessity" was used as an excuse for the relocation and internment. Myer was an essential part of the government efforts to justify the actions to the public. And, he was a part of a government that needed to justify those efforts. It was very important to the war effort that the relocation and internment be conducted smoothly, meaning with little violence or undue stress, especially to the Americans not being relocated or interned. Also, the "regular" citizens needed to feel that these actions were absolutely necessary. (Robinson, 2001. 43, 109-11, 139-41)
The phrase "military necessity" was used by Lt. General DeWitt frequently, and even other administrative officials grew accustomed to using it. Secretary of War Henry Stimson used it in a letter to DeWitt that cited the "urgent military necessity" that mentioned how it was crucial for Japanese and Japanese Americans to engage in prison-labor style chain gangs and pick cotton in Arizona. Ostensibly, this was necessary for parachute production. (Hayahsi, 78)
Military necessity became the politically correct phrase used as an umbrella term for the nearly overt racism driving the decisions to evacuate and relocate and finally intern the Japanese and Japanese Americans in 1942 (Muller 92-3, 117, 133-5, 140). DeWitt was not necessarily the creator of this doctrine, but he did carry it out to the letter. Eventually the entire government institution responsible for the care of the Japanese and Japanese Americans, the War Relocation Authority, was operating under the assumption that this wartime segregation was absolutely necessary for the continued protection of the United States and its citizens - at least those that weren't of Japanese descent.
Military necessity was finally overturned in December of 1944. It took the United States Supreme Court decision to officially make these actions unconstitutional.