Thomas Fairfax, 3rd Lord Fairfax of Cameron, also known as Sir Thomas Fairfax, was an English politician, general and Parliamentary commander-in-chief during the English Civil War. An adept and talented commander, Fairfax led Parliament to many victories, including the crucial Battle of Naseby, effectively becoming military ruler of England, but he was eventually overshadowed by his subordinate Oliver Cromwell, who was more politically adept and radical in action against Charles I. Fairfax became dissatisfied with Cromwell's policy and publicly refused to take part in Charles's show trial. Eventually he resigned, leaving Cromwell to control the country. Because of this, as well as his honourable battlefield conduct and active role in the Restoration of the monarchy after Cromwell's death, he was exempted from the retribution that was exacted on many other leaders of the revolution.

Roundheads were the supporters of the Parliament of England during the English Civil War (1642–1651). Also known as Parliamentarians, they fought against King Charles I of England and his supporters, known as the Cavaliers or Royalists, who claimed rule by absolute monarchy and the principle of the divine right of kings. The goal of the Roundheads was to give to Parliament the supreme control over executive administration of the country/kingdom.