Complex sentence: Henry Whipple, the Episcopal bishop of Minnesota and a reformer of U.S. policies toward Native Americans, first wrote an open letter and then went to Washington DC in the Fall of 1862 to urge Lincoln to proceed with leniency.
Split into simpler sentences: Henry Whipple, the Episcopal bishop of Minnesota and a reformer of U.S. policies toward Native Americans, responded by publishing an open letter. He also went to Washington DC in the Fall of 1862 to urge Lincoln to proceed with leniency.