William Edgar Borah (June 29, 1865 – January 19, 1940) 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.

Borah was born in rural Illinois to a large farming family. He studied at the University of Kansas and became a lawyer in that state before seeking greater opportunities in Idaho. He quickly rose in the law and in state politics, and after a failed run for the House of Representatives in 1896 and one for the United States Senate in 1903, was elected to the Senate in 1907. Before he took his seat in December of that year, he was involved in two prominent legal cases. One, the murder conspiracy trial of Big Bill Haywood, gained Borah fame though Haywood was found not guilty and the other, a prosecution of Borah for land fraud, made him appear a victim of political malice even before his acquittal.

In the Senate, Borah became one of the progressive insurgents who challenged President William Howard Taft's policies, though Borah refused to support former president Theodore Roosevelt's third-party bid against Taft in 1912. Borah reluctantly voted for war in 1917 and, once it concluded, he fought against the Versailles treaty, and the Senate did not ratify it. Remaining a maverick, Borah often fought with the Republican presidents in office between 1921 and 1933, though Calvin Coolidge offered to make Borah his running mate in 1924. Borah campaigned for Herbert Hoover in 1928, something he rarely did for presidential candidates and never did again.

Deprived of his post as Chairman of the Senate Foreign Relations Committee when the Democrats took control of the Senate in 1933, Borah agreed with some of the New Deal legislation, but opposed other proposals. He ran for the Republican nomination for president in 1936, but party regulars were not inclined to allow a longtime maverick to head the ticket. In his final years, he felt he might be able to settle differences in Europe by meeting with Hitler; though he did not go, this has not enhanced his historical reputation. Borah died in 1940; his statue, presented by the state of Idaho in 1947, stands in the National Statuary Hall Collection.

William Edgar Borah was born in Jasper Township, Illinois, near Fairfield in Wayne County. His parents were farmers Elizabeth (West) and William Nathan Borah. Borah was distantly related to Katharina von Bora, the Catholic nun who left her convent in the 16th century and married reformer Martin Luther. His Borah ancestors came to America in about 1760, fought in the Revolutionary War, and moved west with the frontier. The young William E. Borah was the seventh of ten children, and the third son.

Although Borah was not a good student, at an early age he began to love oratory and the written word. Borah was educated at Tom's Prairie School, near Fairfield. When Borah exhausted its rudimentary resources, his father sent him in 1881 to Southern Illinois Academy, a Cumberland Presbyterian academy at Enfield, to train for the ministry. The 63 students there included two future U.S. senators, Borah and Wesley Jones, who would represent the state of Washington; the two often debated as schoolboys. Instead of becoming a preacher, Borah was expelled in 1882 for hitching rides on the Illinois Central to spend the night in the town of Carmi.

He ran away from home with an itinerant Shakespearean company, but his father persuaded him to return. In his late teenage years, he became interested in the law, and later stated, "I can't remember when I didn't want to be a lawyer ... there is no other profession where one can be absolutely independent".

With his father finally accepting his ambition to be a lawyer rather than a clergyman, Borah in 1883 went to live with his sister Sue in Lyons, Kansas; her husband, Ansel M. Lasley, was an attorney. Borah initially worked as a teacher, but became so engrossed in historical topics at the town library that he was ill-prepared for class; he and the school parted ways. In 1885 Borah enrolled at the University of Kansas, and rented an inexpensive room in a professor's home in Lawrence; he studied alongside students who would become prominent, such as William Allen White and Fred Funston. Borah was working his way through college, but his plans were scuttled when he contracted tuberculosis in early 1887. He had to return to Lyons, where his sister nursed him to health, and he began to read law under his brother-in-law Lasley's supervision. Borah passed the bar examination in September 1887, and went into partnership with his brother-in-law.