Theodore Roosevelt, Jr. (/ˈroʊzəvɛlt/; October 27, 1858 – January 6, 1919), also known as T.R., and to the public (but never to friends and intimates) as Teddy, was the twenty-sixth President of the United States, and a leader of the Republican Party and of the Progressive Movement. He became President of the United States at the age of 42. He served in many roles including Governor of New York, historian, naturalist, explorer, author, and soldier. He is most famous for his personality: his energy, his vast range of interests and achievements, his model of masculinity, and his "cowboy" persona. As Assistant Secretary of the Navy, Roosevelt prepared for and advocated war with Spain in 1898. He organized and helped command the 1st U.S. Volunteer Cavalry Regiment, the Rough Riders, during the Spanish-American War. Returning to New York as a war hero, he was elected governor. He was a professional historian, a lawyer, a naturalist and explorer of the Amazon Basin; his 35 books include works on outdoor life, natural history, the American frontier, political history, naval history, and his autobiography.