This article documents the career of Winston Churchill in Parliament from its beginning in 1900 to the start of his term as Prime Minister of the United Kingdom in World War II. Churchill entered Parliament as member for Oldham in 1900 as a Conservative. He changed parties in 1904 after increasing disagreement with the mainstream Conservative policy of protectionist tariffs preferentially favouring trade with the British Empire, joining the Liberals and winning the seat of Manchester North West. His political ascent was rapid; he became, successively, Under-Secretary of State for the Colonies, President of the Board of Trade, Home Secretary, and First Lord of the Admiralty, all before he was 40 years old.