During WWII 11,507 Germans were detained under the Justice Department's Alien Enemy Control Program (as compared to 110+ thousand Japanese).

This did not stop other German born Americans from being able to join the military and fight against the Nazis though. My grandfather was born in Germany and immigrated at the age of ~6. He served in the war effort on the Western Front. It is unknown how many German born Americans served in the US military at the time.

https://en.wikipedia.org/wiki/Internment_of_German_Americans