WDC's operational region covered the states of Washington, Oregon, California, Idaho, Montana, Nevada, Utah, and Arizona, and the Territory of Alaska.

The exclusion zone would later be expanded to include the entire state of California[4] and Executive Order 9066 also encompassed Alaska in the process.

On 19 December 1941, General DeWitt had recommended to the Army's GHQ "that action be initiated at the earliest practicable date to collect all alien subjects fourteen years of age and over, of enemy nations and remove them to the Zone of the Interior".

He initially felt very differently about the necessity and practicality of locking up citizens as well, in a telephone conversation with Major General Allen W. Gullion on 26 December.

Regardless of this, following the Roberts Commission report of 25 January 1942 accusing persons of Japanese ancestry of widespread espionage in Hawaii prior to Pearl Harbor, along with his perception of public opinion as anti-Japanese, he became a proponent of internment of Japanese and initially German and Italian-descended persons.