Theodore Roosevelt was a leader.
Theodore Roosevelt . As a leader of the Republican Party during this time , he became a driving force for the Progressive Era in the United States in the early 20th century . Roosevelt served as Assistant Secretary of the Navy under President William McKinley , but resigned from that post to lead the Rough Riders during the Spanish -- American War . As a leader of the Progressive movement , he championed his `` Square Deal '' domestic policies , promising the average citizen fairness , breaking of trusts , regulation of railroads , and pure food and drugs .