In June 1866—a year after the American Civil War—an 800-man contingent of the Irish Fenian Brotherhood invaded Canada from Buffalo, New York, to free Ireland from British rule. The force was led by Irish-born John Charles O’Neill, a veteran of the 5th Indiana Cavalry. The three-day invasion was a military success but a political failure. Yet the 28-year old O’Neill was celebrated for his leadership and humanity. Elevated to the presidency of the Brotherhood, “General” John O’Neill would again lead Irish nationalists against Canada in 1870. Jailed and later pardoned by President U.S. Grant, O’Neill left the Fenians and attempted a third, futile attack into Canada from North Dakota. His revolutionary career at an end, O’Neill became a colonizer, urging Irish Americans to abandon urban communities in the East to settle on the fertile plains of the West. O’Neill City, Nebraska, is named in his honor. This first full-length biography covers the rise, fall and resurgence of a remarkable figure in American and Irish history.