William Edgar Borah was an outspoken Republican politician, one of the best-known figures in Idaho's history. A progressive who served as a United States senator from 1907 until his death in 1940, Borah voted for American entry into World War I but is often considered an isolationist, for he led the Irreconcilables, senators who opposed ratification of the Treaty of Versailles, which would have made the U.S. part of the League of Nations.