Naseby, Battle of, June 14, 1645, the decisive action of the English Civil War, or Great Rebellion. The Parliamentary army of about 14,000, led by Sir Thomas Fairfax, defeated about 8,000 Royalists, commanded by King Charles I, near the Northamptonshire village of Naseby. Oliver Cromwell's cavalry struck the decisive blow. The Royalists lost nearly all their arms and artillery, and thousands of their men were taken prisoner.