Theodore Roosevelt entered elected politics in the New York State Assembly in 1882, advocating civil service reform and challenging corruption. After personal bereavements in 1884, he spent substantial time ranching in Dakota Territory while continuing to write about history and natural life. His later public appointments included the United States Civil Service Commission, leadership of the New York City Police Board, and service as assistant secretary of the navy.

When war with Spain began in 1898, Roosevelt resigned his naval post and helped organize the Rough Riders, the First United States Volunteer Cavalry. His service in Cuba made him a national celebrity and helped him win the governorship of New York. Elected vice president on William McKinley's ticket in 1900, he became president after McKinley's assassination in September 1901. He won election in his own right in 1904.

Roosevelt's Square Deal sought regulation of powerful corporations and greater protection for consumers. His administration prosecuted the Northern Securities combination, intervened in the 1902 coal strike, and supported stronger railroad regulation. The Pure Food and Drug Act and Meat Inspection Act became law in 1906. Conservation was another major commitment: he expanded national forests, wildlife refuges, and protected public lands, using the Antiquities Act to designate national monuments. These policies helped establish a durable federal conservation role, although their development also reflected the unequal power of Indigenous communities and the state.

Abroad, Roosevelt promoted American strategic strength and intervention. He supported Panama's separation from Colombia and obtained canal rights, while his corollary to the Monroe Doctrine asserted a wider American policing role in the hemisphere. Mediation of the Russo-Japanese War earned him the Nobel Peace Prize in 1906. His ideas of national progress included racial hierarchies and imperial ambitions that limit any account of his reforms as universal equality. After leaving office in 1909, he broke with William Howard Taft and ran as the Progressive Party's presidential candidate in 1912, surviving an assassination attempt during the campaign. Defeated by Woodrow Wilson, he continued writing, exploring, and arguing for military preparedness and entry into the First World War. He died in 1919, leaving a presidency that enlarged federal power in domestic regulation, conservation, and foreign affairs.