Sidney or Sydney, Algernon, a noted politician and soldier of extreme republican views, second son of Robert, second Earl of Leicester; first came into public notice in 1641-1642 by his gallant conduct as leader of a troop of horse in the Irish Rebellion; came over to England in 1643, joined the Parliamentarians, rose to a colonelcy and command of a regiment in 1645; was subsequently governor of Dublin and of Dover (1647), entered Parliament (1646), and although appointed one of the commissioners to try Charles I., absented himself from the proceedings, but afterwards approved of the execution; withdrew from politics during Cromwell's Protectorate, but on the reinstating of the Long Parliament (1659) became a member of the Council of State; was on a diplomatic mission to Denmark when the Restoration took place, and till his pardon in 1677 led a wandering life on the Continent; intrigued with Louis XIV. against Charles II., assisted William Penn in drawing up the republican constitution of Pennsylvania, was on trumped-up evidence tried for complicity in the Rye House Plot and summarily sentenced to death by Judge Jeffreys, the injustice of his execution being evidenced by the reversal of his attainder in 1689 (1622-1683).