In June 1866, an 800-man contingent of the Irish Fenian Brotherhood invaded Canada from Buffalo, New York, in an effort to free Ireland from British rule. The force was led by Irish-born John Charles O'Neill, a veteran of the Union Army's 5th Indiana Cavalry. The three-day invasion was a military success but a political failure, yet O'Neill was celebrated for his leadership and humanity. Elevated to the presidency of the Fenian Brotherhood, "General" 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. O'Neill then became a colonizer, urging Irish Americans to abandon cities 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.