Complex sentence: After he left office, Harrison visited the World's Columbian Exposition in Chicago in June 1893, where the nation's first commemorative postage was introduced, an initiative of his Postmaster General, John Wanamaker.
Split into simpler sentences: After he left office, Harrison visited the World's Columbian Exposition in Chicago in June 1893, where the nation's first commemorative postage was introduced. It was an initiative of his Postmaster General, John Wanamaker.