Theodore Roosevelt was the 26th President of the United States. He had been the 25th Vice President before becoming President upon the assassination of President William McKinley. Inaugurated at the age of 42, Roosevelt became the youngest President in U.S. history. Within the Republican Party, he was a reformer who sought to bring the party's conservative ideals into the 20th century. He broke with his friend and successor William Howard Taft and ran as a third party candidate in 1912 on the Progressive Party ticket. Before his presidency, Roosevelt served as a New York State assemblyman, Police Commissioner of New York City, U.S. Civil Service Commissioner, and Assistant Secretary of the Navy. As a colonel, he commanded his famous all-volunteer First U.S. National Cavalry regiment, the "Rough Riders" during the Spanish–American War. Roosevelt also served a successful term as Governor of New York. He was a famous historian and naturalist; his 15 books include works on outdoor life, natural history, U.S. Western and political history, an autobiography and a host of other topics. In his lifetime, he was considered a foremost authority on North American big game animals and Eastern birds.