German and Italian Americans Were Spared Internment: The U.S. Military Refused It

A declassified archive of a U.S. Army book explains in-depth the mechanics of rounding up Japanese Americans for internment camps.

What stands out is the Army’s strong objections to rounding up any nationality group.

Politicians and the public wanted resident aliens and naturalized citizens from Japan, Germany, and Italy put in concentration camps. 

The Army relented in the matter of Japanese because they had to obey the Commander-in-Chief—and for practical reasons, they argued that Japanese were more concentrated than Italians and Germans. 

But the Army's leading general (Gen. DeWitt) in the matter also believed that Japanese in the U.S. presented no military threat as a Fifth Column.

For more, see https://history.army.mil/books/wwii/Guard-US/ch5.htm.

Here is an excerpt:

CHAPTER V Japanese Evacuation From the West Coast

One of the Army's largest undertakings in the name of defense during World War II was the evacuation of almost all persons of Japanese ancestry from California, from the western halves of Oregon and Washington, and from southern Arizona. 

Initial plans for evacuation of suspected persons from strategic areas along the Pacific front concerned enemy aliens of all three Axis nations— Germany, Italy, and Japan-rather than persons of Japanese ancestry alone. 

Of the latter, the census of 1940 showed that, out of a total of 126,947 in the continental United States, 112,353 were living in the three Pacific states. California alone had 93,717 Japanese, or nearly three-fourths of the national total. Of the west coast Japanese, 40,869 were aliens ineligible for citizenship, and 71,484 were American-born citizens.

In early 1942 there were about 58,000 Italian and 22,000 German aliens in the Pacific states. Most of the Germans, and a large proportion of the Japanese and Italians, lived in or near the principal cities and adjacent strategic areas. 

A good many of the German aliens were recent refugees from Nazi Germany. In contrast to the Germans and Italians, the Japanese in the Pacific states, and especially in California, had been the target of hostility and restrictive action for several decades, a factor that unquestionably colored the measures taken against these people after Pearl Harbor.

…

A prewar agreement made the Department of Justice responsible for controlling enemy aliens in the continental United States in the event of war. During 1941 this department (primarily, through its Federal Bureau of Investigation) scrutinized the records of prospective enemy aliens and compiled lists of those against whom there were grounds for suspicion of disloyalty.  

By 13 December the Department of justice had interned a total of 831 alien residents of the Pacific states, including 595 Japanese and 187 Germans, and by 16 February 1942 the number of alien Japanese apprehended had increased to 1,266.

During the first few days after the Pearl Harbor attack the west coast was greatly alarmed by a number of reports-all false-of enemy ships offshore, and it was in this atmosphere that the first proposal for a mass evacuation of the Japanese developed. On 10 December an agent of the Treasury Department reported to Army authorities that "an estimated 20,000 Japanese in the San Francisco metropolitan area were ready for organized action." 

However General DeWitt may have felt during December about the treatment of enemy aliens, he was then firmly opposed to any evacuation of citizens. 

In a telephone conversation he had on 26 December with Maj. Gen. Allen W. Gullion, the Provost Marshal General, the latter remarked that he had just been visited by a representative of the Los Angeles Chamber of Commerce, who had asked for a roundup of all Japanese in the Los Angeles area.

In response, General DeWitt said (and General Gullion expressed agreement with what he said):

I thought that thing out to my satisfaction .... if we go ahead and arrest the 93,000 Japanese, native born and foreign born, we are going to have an awful job on our hands and are very liable to alienate the loyal Japanese from disloyal .... I'm very doubtful that it would be common sense procedure to try and intern or to intern 117,000 Japanese in this theater .... I told the governors of all the states that those people should be watched better if they were watched by the police and people of the community in which they live and have been living for years .... and then inform the F.B.I. or the military authorities of any suspicious action so we could take necessary steps to handle it . . . rather than try to intern all those people, men, women and children, and hold them under military control and under guard. I don't think it's a sensible thing to do .... I'd rather go along the way we are now . . . rather than attempt any such wholesale internment . . . . An American citizen, after all, is an American citizen. And while they all may not be loyal, I think we can weed the disloyal out of the loyal and lock them up if necessary.