On Saturday night, Washington National Opera unveiled the first installment, Das Rheingold, of its new staging of Richard Wagner’s operatic tetralogy Der Ring des Nibelungen at the Kennedy Center Opera House. A co-production with San Francisco Opera, this “American Ring cycle” is the work of director Francesca Zambello and a team of American artistic collaborators. They have brought together images drawn from the history of the United States to transform Wagner’s libretto, based on pre-Christian…