German nationals were automatically classified as "enemy aliens". I know because the uncle of my best friend Nancy Klug was taken away in handcuffs only because he had emigrated a few years before - according to Wikipedia, during WWII, the United States detained a total of 11,507 ethnic Germans, out of 1,237,000 of those identified as not being US citizens and of German birth.
The US detained only a few who were Italian nationals, (not US citizens, or long-term US residents). Representing the axis powers, there were some 695,000 Italian 'nationals' in the United States. Taken into custody, along coastal areas were 1881 Italian nationals detained under wartime restrictions.
If you remember we were whipped up in fear about Asians and we were urged to burn and trash any piece of ceramics we owned that said made in either Japan or China. I also remember the fear was urged reminding us how easily what happened in Pearl Harbor could happen to cities on the west coast.
I think that was the difference why German's or Italians were not detained in similar numbers - neither attacked US soil. However, for sure German's were detained as was merchant seamen either German or Italian trapped in US ports by the outbreak of war. At the time my father had something to do with shipping and I remember him coming home and telling us of the FBI round up of Germans aboard ships unloading cargo on the docks.
What is different - where Japanese interns were compensated and received an apology from the United States, German and Italians received an apology but no compensation.
However, prior to the outbreak of WWII Japanese adults were not allowed to become citizens, only children born in the US of Japanese heritage where as, this did not hold true for Germans or Italians. Another tidbit Italians had been urged to become citizens through some process that was as simple as visiting an office and not having to study or take a test.