The First English Civil War was fought in England and Wales from approximately August 1642 to June 1646 and forms part of the 1638 to 1651 Wars of the Three Kingdoms. Other related conflicts include the Bishops' Wars, the Irish Confederate Wars, the Second English Civil War, the Anglo-Scottish war (1650–1652) and the Cromwellian conquest of Ireland. Based on modern estimates, 15% to 20% of all adult males in England and Wales served in the military between 1638 to 1651 and around 4% of the total population died from war-related causes, compared to 2.23% in World War I. These figures illustrate the impact of the conflict on society in general and the bitterness it engendered.

Political conflict between Charles I and Parliament dated back to the early years of his reign and culminated in the imposition of Personal Rule in 1629. Following the 1639 to 1640 Bishops' Wars, Charles recalled Parliament in November 1640 hoping to obtain funding that would enable him to reverse his defeat by Scots Covenanters but in return they demanded major political concessions. While the vast majority supported the institution of monarchy, they disagreed on who held ultimate authority; Royalists generally argued Parliament was subordinate to the king, while most of their Parliamentarian opponents backed constitutional monarchy. However, this simplifies a very complex reality; many initially remained neutral or went to war with great reluctance and the choice of sides often came down to personal loyalties.

When the conflict began in August 1642, both sides expected it to be settled by a single battle, but it soon became clear this was not the case. Royalist successes in 1643 led to an alliance between Parliament and the Scots who won a series of battles in 1644, the most significant being the Battle of Marston Moor. In early 1645, Parliament authorised the formation of the New Model Army, the first professional military force in England, and their success at Naseby in June 1645 proved decisive. The war ended with victory for the Parliamentarian alliance in June 1646 and Charles in custody, but his refusal to negotiate concessions and divisions among his opponents led to the Second English Civil War in 1648.