President Franklin D. Roosevelt issued Executive Order 9066, giving authority to restrict military sensitive locations.

His father, Brigadier General Calvin DeWitt (1840–1908), served with the United States Army and was an 1863 graduate of Princeton University.

[3] Between 1919 and 1930, he served in various quartermaster positions, including assistant commandant of the General Staff College, Chief of the Storage and Issue Branch, and the Supply Division.

He also assumed control of the Gold Star Mothers' Pilgrimage to visit the graves of their sons who died in France during the First World War.

At age 62, DeWitt would produce the "Final Report: Japanese Evacuation from the West Coast, 1942", which argued for the removal and internment of American-born citizens of ancestry tie to a past or present immigrant of Japan.

DeWitt was in San Francisco on the evening of 8 December 1941, one day after the Japanese attack on Pearl Harbor, when air raid sirens were sounded.

At the Civil Defense Council meeting, DeWitt suggested that it might have been a good thing if the planes had dropped bombs to "awaken this city."

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.

"[10] Days later, DeWitt announced that the army had acquired 5,800 acres (23 km2) of land near Manzanar, California, for construction of a "reception center" which he said was "to be used principally as a clearing house for the more permanent resettlement elsewhere for persons excluded from military areas.

On that date, General DeWitt issued new orders applying to Japanese-Americans, setting an 8 p.m. to 6 a.m. curfew and banning ownership of firearms, radios, cameras, and other contraband.

DeWitt stated, "Let me warn the affected aliens and Japanese-Americans that anything but strict compliance with this proclamation's provisions will bring immediate punishment.

A federal judge, James Alger Fee of Portland, Oregon, ruled in November, 1943 that American citizens could not be detained without a proclamation of martial law.

"[16] After the relocation of Japanese Americans was complete, DeWitt lifted curfew restrictions on Italian-Americans on 19 October and on German-Americans on 24 December.

He testified before Congress, in 1943, that he would "use every proper means" at his disposal to stop the resettlement of Japanese Americans outside camp and their eventual return to the West Coast after the war.

His and Colonel Karl Bendetsen's "Final Report" (circulated and then hastily redacted in 1943 and 1944) also laid out his position that their race made it impossible to determine their loyalty, thus necessitating internment.

[4] The earlier, racist and inflammatory version as well as the FBI and Office of Naval Intelligence (ONI) reports led to the coram nobis retrials, which overturned the convictions of Fred Korematsu, Gordon Hirabayashi and Minoru Yasui, on all charges related to their refusal to submit to exclusion and internment.

In the words of Department of Justice officials writing during the war, the justifications were based on "willful historical inaccuracies and intentional falsehoods."

When houses of prostitution were closed across America, General DeWitt allowed Sally Stanford to continue to operate a high-class brothel in San Francisco.

His paternal great-grandfather, John Radcliffe DeWitt (1752–1808), was a captain in the American Revolutionary War and served as a New York State Assemblyman from 1785 to 1788.

His first cousin, twice removed, William Radcliffe DeWitt V, served in the United States Marine Corps as a private during the Korean War.

His first cousin, three times removed, Robert George Schoenkopf III, served as a sergeant in the Marine Corps during the Vietnam War.