Seyed Hossein Mousavian is an ex-Iranian diplomat whose tenure in Germany coincided with the regime’s assassination of four dissidents on German soil. Yet he currently lives in comfort in the United States. Does he belong here? That’s the question raised by a number of prominent Iranian-Americans in a recent letter to Attorney General Bill Barr. The details are grim. Late evening on Sept. 17, 1992, four men were dining together in the backroom of the Mykonos restaurant, a Greek eatery in central Berlin. They were Iranian exiles who had gathered to meet a prominent Kurdish opponent of the Tehran regime....