Theodore Roosevelt was active.
Theodore Roosevelt . 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 . Making conservation a top priority , he established a myriad of new national parks , forests , and monuments intended to preserve the nation 's natural resources .