Roosevelt took office as Vice President in March 1901 , and he assumed the presidency at age 42 following McKinley 's assassination that September ; he remains the youngest person to become President of the United States . He was a leader of the Progressive movement , and he championed his " Square Deal " domestic policies , promising the average citizen fairness , breaking of trusts , regulation of railroads , and pure food and drugs . He made conservation a top priority and established many new national parks , forests , and monuments intended to preserve the nation 's natural resources . In foreign policy , he focused on Central America where he began construction of the Panama Canal . He expanded the Navy and sent the Great White Fleet on a world tour to project the United States ' naval power around the globe . His successful efforts to broker the end of the Russo-Japanese War won him the 1906 Nobel Peace Prize . He avoided controversial tariff and money issues . He was elected to a full term in 1904 and continued to promote progressive policies , many of which were passed in Congress . He groomed his close friend William Howard Taft , and Taft won the 1908 presidential election to succeed him
McKinley 's assassination that September
Assassination of William McKinley
Theodore Roosevelt