Statesman, author, explorer, soldier, naturalist, and reformer who served as the 26th President of the United Statesfrom 1901 to 1909. 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.

Born a sickly child with debilitating asthma, Roosevelt successfully overcame his health problems by embracing a strenuous lifestyle. He integrated his exuberant personality, vast range of interests, and world-famous achievements into a "cowboy" persona defined by robust masculinity. Home-schooled, he began a lifelong naturalist avocation before attending Harvard College. His first of many books, The Naval War of 1812 (1882), established his reputation as both a learned historian and as a popular writer. Upon entering politics, he became the leader of the reform faction of Republicans in New York's state legislature. Following the deaths of his wife and mother, he took time to grieve by escaping to the wilderness of the American West and operating a cattle ranch in the Dakotas for a time, before returning East to run unsuccessfully for Mayor of New York City in 1886. He served as Assistant Secretary of the Navyunder William McKinley, resigning after one year to serve with the Rough Riders, where he gained national fame for courage during the Spanish–American War. Returning a war hero, he was elected governor of New York in 1898. The state party leadership distrusted him, so they took the lead in moving him to the prestigious but powerless role of vice president as McKinley's running mate in the election of 1900. Roosevelt campaigned vigorously across the country, helping McKinley's re-election in a landslide victory based on a platform of peace, prosperity, and conservatism.

Following the assassination of President McKinley in September 1901, Roosevelt, at age 42, succeeded to the office, becoming the youngest United States President in history. Leading his party and country into the Progressive Era, he championed his "Square Deal" domestic policies, promising the average citizen fairness, breaking of trusts, regulation of railroads, and pure food and drugs. Making conservation a top priority, he established a myriad of 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 greatly expanded the United States 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 end the Russo-Japanese War won him the 1906 Nobel Peace Prize.