Louis XVI of France, who was guillotined in 1793 during the Revolution and Reign of Terror, is commonly portrayed in fiction and film either as a weak and stupid despot in thrall to his beautiful, shallow wife, Marie Antoinette, or as a cruel and treasonous tyrant. Historian John Hardman disputes both these versions in a fascinating new biography of the ill-fated monarch. Based in part on new scholarship that has emerged over the past two decades,...