Why were there no internment camps for German-American citizens in USA during WW2?

What you are missing is that not all Japanese citizens were interned, only those in Hawaii and California and the west coast, i.e., the places the Japanese could attack. If Germany had raided an American military base and done significant damage, I suspect we would have started rounding up Germans as well.

In hindsight, the Japanese threat to the American mainland was almost non-existent, but after the attack on Pearl Harbor, which killed or wounded over 3,500 men and sank 8 battleships the US Congress was in no mood to treat the threat of Japanese espionage lightly. The Japanese knew exactly the disposition of entire US Fleet and the strength of its defenses due to espionage by Japanese-American citizens in Hawaii.

Japanese men located in foreign countries often spied for the country of their origin. For example, in one notorious case there was a Japanese shopkeeper who ran a photography business in Singapore, the key British naval base in the Far East. It was found out after the war that this man had secretely and systematically made duplicates of every single roll of film brought to him for development and sent the duplicate prints back to Japan. By using these thousands of photographs the Japanese were able to construct a detailed model of the fortress of Singapore down to the last storm drain and gun emplacement, even of all the secret areas. These models and diagrams were enormously useful to them during the Battle of Singapore in which they defeated the British and took control of Indonesia and the surrounding islands, including Java, an important source of oil.

This was the kind of situation the internments were intended to prevent. Of course, it is debatable the justice of imprisoning many thousands of people to prevent the work of a few dozen spies. Though, remember that it is easy now for you to be self-righteous after we have won the war, in 1941 it was not clear that war was won yet. What if YOU had ended up in a Japanese camp, instead of the other way around, like many Americans including the entire Phillipine army did. It's easy to be generous and forgiving after you have won, but when you are fighting for your life and you or your family may be killed, things are different.

I would like to bring up a few pertinent pieces of information which haven't been discussed yet. It shouldn't be just a Japanese / German comparison. There were three great powers in the Axis.. Japan, Germany and Italy.

(1) Japan Attacked the United States at Pearl Harbor not Germany or Italy. Most Americans took that much more personally than did the United States Government at the time. Today we think of industrialized Germany as the more existential threat and perhaps it was. As the United States government sided with the British Government and agreed to fight the War in Europe first. But most Americans in 1942 thought of Japan as the real enemy. Japan had attacked the US first in a sneak attack without a declaration of war. We were in the war because of Japan, not Germany or Italy. It was Japan's fanaticism which inspired the most fear/concern/hatred among the American public during WWII. It is a modern contrivance which benefits from 20/20 hind site, that we think of Germany as the Primary enemy, that's not how the American public felt during the war.

(2) After the Attack on Pearl Harbor it had been three Japanese American who aided a captured Japanese flyer (Shigenori Nishikaichi) to escape captivity. This was also covered widely in the press and placed into question the loyalty of all ethnic Japanese over American nationalism. The Niihau incident. When you think of most of the Pacific fleet being destroyed at Pearl harbor and many Americans fearing an invasion of Hawaii or even California, the sizable Japanese American populations seemed like a more legitimate concern given both the sneak attack and then the aid given to the Japanese flyer by previously unaquanted Japanese American citizens.

(3) When the United States entered WWI there was great fear of German 5th columnists. In 1916 the German Foreign Minister (Arthur Zimmerman) threatened the American Ambassador to Germany (James Gerald). That Germany had a half million Germans in the United States which would rise up with pro-German Irish and overthrow the US government if the US entered the war. To which the US Ambassador responded that the US had half a million lamp posts to hang those German revolutionaries from. Still this was a very serious concern during WWI. The US entered WWI in April 6th 1917 and no significant mass fifth column ever materialized. So when WWII occurred, German American patriotism was another generation embedded, had already been tested, and wasn't nearly as scary to the American public as Japanese fanaticism. There were so many Germans and Italians in the United States most Americans were much better aquatinted with them and their patriotism.

On a partly related note there were World War II prisoner-of-war camps in the U.S., some of then exclusively for German soldiers: Camp Trinidad in Colorado was a case in point.

Kurt Landsberger was a Jewish refugee from Nazi Germany who was drafted into the U.S. army and served as interpreter at Camp Trinidad. He used a somewhat odd suffix in the title of his book about Camp Trinidad, presumably to point out that the inmates were mostly left without harm (despite, as the book tells, at least two were shot on an attempted escape):

Kurt Landsberger: Prisoners of War at Camp Trinidad, Colorado 1943 - 1946: Internment, Intimidation, Incompetence and Country Club Living

While it seems obvious that naturalized immigrants from an enemy country and prisoners of war require different attention and treatment in terms of national security, Landsberger's fate and book may still provide the key to (one aspect of an answer to the) question: if the U.S. would have interned German-Americans, how would it have made the presumably necessary distinction between Jewish refugees from Germany (many of them born there and with German passports) and the "evil" sort of Germans?

Presumably any distinction on a large scale would have been hard to administer, and the supreme court might not have had it even during times of war.

1. The question was about US Citizens of German and Italian descent. Most of the answers above was about resident aliens and visitors from Germany and Italy.

2. I suppose that the correct answer is two part: (a) there were too many US Citizens of German and Italian descent (today is ~ 1/6th of the total population) vs 100k US Citizens of Japanese descent, (b) for some reason (I suspect pure racist one) society expected that German- and Italian-Americans would be way more loyal to US than Japanese-Americans.