Theodore "Teddy" Roosevelt (October 27, 1858 – January 6, 1919; pronounced /ˈroʊzəvɛlt/ ROE-zə-velt) was the 26th President of the United States. He is well remembered for his energetic personality, range of interests and achievements, leadership of the Progressive Movement, model of masculinity, and his "cowboy" image. He was a leader of the Republican Party and founder of the short-lived Progressive ("Bull Moose") Party of 1912. Before becoming President (1901–1909) he held offices at the municipal, state, and federal level of government. Roosevelt's achievements as a naturalist, explorer, hunter, author, and soldier are as much a part of his fame as any office he held as a politician.

Born to a wealthy family, Roosevelt was an unhealthy child suffering from asthma who stayed at home studying natural history. In response to his physical weakness, he embraced a strenuous life. He attended Harvard, where he boxed and developed an interest in naval affairs. A year out of Harvard, in 1881 he ran for a seat in the state legislature. His first historical book, The Naval War of 1812, published in 1882, established his reputation as a serious historian. After a few years of living in the Badlands, Roosevelt returned to New York City, where he gained fame for fighting police corruption. He was effectively running the US Department of the Navy when the Spanish American War broke out; he resigned and led a small regiment in Cuba known as the Rough Riders, earning himself a nomination for the Medal of Honor. After the war, he returned to New York and was elected Governor; two years later he was nominated for and elected Vice President of the United States.

In 1901, President William McKinley was assassinated, and Roosevelt became president at the age of 42, taking office at the youngest age of any U.S. President in history. Roosevelt attempted to move the Republican Party in the direction of Progressivism, including trust busting and increased regulation of businesses. Roosevelt coined the phrase "Square Deal" to describe his domestic agenda, emphasizing that the average citizen would get a fair shake under his policies. As an outdoorsman and naturalist, he promoted the conservation movement. On the world stage, Roosevelt's policies were characterized by his slogan, "Speak softly and carry a big stick". Roosevelt was the force behind the completion of the Panama Canal; he sent out the Great White Fleet to display American power, and he negotiated an end to the Russo-Japanese War, for which he won the Nobel Peace Prize. Roosevelt is the first American to win the Nobel Peace Prize.