Theodore Roosevelt Jr., often referred to as Teddy or by his initials, T. R., was an American politician, statesman, soldier, conservationist, naturalist, historian, and writer who served as the 26th president of the United States from 1901 to 1909. He previously served as the 25th vice president under President William McKinley from March to September 1901 and as the 33rd governor of New York from 1899 to 1900. Assuming the presidency after McKinley's assassination, Roosevelt emerged as a leader of the Republican Party and became a driving force for anti-trust and Progressive policies.