On May 18, 1905, Frederick A. Kerry, a 32-year-old Viennese, arrived in New York City by steamship, the Königin Luise, with his wife and 4-year-old son, hopeful that his new country would bring him the success and social acceptance that had eluded him in Europe. Mr. Kerry probably could not have imagined that within a century a grandson, John Forbes Kerry, would find himself the Democratic candidate for president. Frederick Kerry brought with him a secret: he was born a Jew, Fritz Kohn, in what is now the Czech Republic, but he and his wife, Ida, had converted to Roman...