Algernon Sidney was a prominent English politician, republican political theorist, and army officer during the 17th century. A staunch opponent of absolute monarchy, he became a key figure in the resistance against King Charles II. His most influential work, 'Discourses Concerning Government,' argued for popular sovereignty and the right to overthrow a tyrannical government. Accused of treason for his political writings and alleged involvement in the Rye House Plot, Sidney was executed and became celebrated as a martyr for liberty.