Theodore Roosevelt was active.
Theodore Roosevelt . He also served as the 25th Vice President of the United States and as the 33rd Governor of New York . 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 . Following McKinley 's assassination in September 1901 , Roosevelt became president at age 42 , and remains the youngest president . In foreign policy , he focused on Central America , where he began construction of the Panama Canal .