Complex sentence: P. T. Barnum was considered the father of modern day advertising, and one of the most famous showmen/managers for Freak Show performers, in the United States was a major figure in popularizing the entertainment.
Split into simpler sentences: P. T. Barnum was considered the father of modern day advertising, and one of the most famous showmen/managers of the Freak Show industry. In the United States he was a major figure in popularizing the entertainment.