As a young Republican, he held a number of political posts in New York in the 1880s and 1890s, and was a leader of reform Republicans in the state. Appointed assistant secretary to the U.S. Navy in 1897, he vehemently advocated war with Spain. When the Spanish-American War began in 1898, he helped Col. Leonard Wood organize and lead the "Rough Riders," a volunteer cavalry that became famous for its contribution to the United States victory at the Battle of San Juan Hill in Cuba. The publicity-minded Roosevelt rode his military fame to the New York governor's seat in 1898 and to the U.S. vice presidency in 1900.
In 1901, President William McKinley was assassinated, and Roosevelt, at 43 years old, became the youngest president ever to assume the office. He stamped the presidency with a vitality that delighted most Americans and was elected to a second term in 1904. Abroad, Roosevelt was an expansionist who asserted his executive powers to defend U.S. interests. At home, he sought to balance the interests of farmers, workers, and the business class. He insisted on a strong navy, encouraged the independence of Panama and the construction of the Panama Canal, promoted the regulation of trusts and monopolies, and set aside land for America's first national parks and monuments. In 1906, he was awarded the Nobel Peace Prize for his mediation of the negotiations that ended the Russo-Japanese War.
In 1912, three years after finishing his second term, Roosevelt ran for president again as the candidate of the new Progressive Party, which was made up reform-minded Republicans dissatisfied with the Republican Party leadership. Challenging President, President William Howard Taft, he campaigned on his "Square Deal" platform of social reform. In November, the divided Republican Party was defeated by Democrat Woodrow Wilson.
In the last few years of his life, Roosevelt became a vocal advocate of the U.S. entrance into World War I and even sought to win a commission to lead a U.S. Army division in Europe. President Wilson declined, and after the war Roosevelt opposed Senate ratification of Wilson's League of Nations. On January 6, 1919, Roosevelt died at Sagamore Hill, his estate overlooking New York's Long Island Sound. The tropical diseases he contracted during his many travels had likely caught up with him. He was 60 years old.