The US doesnt have jany legal authority or jurisdiction to charge and put on trial individuals for crimes committed outside the United States unless the crimes were committed against American citizens.
Instead, what the DoJ does is charge anyone with a past Nazi association for violating U.S. immigration laws. A sample immigration questionnaire from the time:
Did you, during the period March 23, 1933 to May 8, 1945, in association with either the Nazi Government of Germany or any organization or government associated or allied with the Nazi Government of Germany, ever order, incite, assist or otherwise participate in the persecution of any person because of race, religion, national origin or political opinion.
Yes...... No....
Immigration Questionnaire (pdf)
The guy violated US immigration law.
I think God of Thunder proposed a sensible solution.