The Battle of Roundway Down was fought on 13 July 1643 near Devizes, in Wiltshire during the First English Civil War. Despite being outnumbered and exhausted after riding overnight from Oxford, a Royalist cavalry force under Lord Wilmot won a crushing victory over the Parliamentarian Army of the West under Sir William Waller.

Viewed as their most decisive victory of the war, the Royalists secured control of South West England which they held until late 1645. Two weeks later they captured the port of Bristol, allowing them to establish links with supporters in Ireland.

The English Civil War (1642–1651) was a series of civil wars and political machinations between Parliamentarians ("Roundheads") and Royalists ("Cavaliers"), mainly over the manner of England's governance and issues of religious freedom. It was part of the wider Wars of the Three Kingdoms. The first (1642–1646) and second (1648–1649) wars pitted the supporters of King Charles I against the supporters of the Long Parliament, while the third (1649–1651) saw fighting between supporters of King Charles II and supporters of the Rump Parliament. The wars also involved the Scottish Covenanters and Irish Confederates. The war ended with Parliamentarian victory at the Battle of Worcester on 3 September 1651.

Unlike other civil wars in England, which were mainly fought over who should rule, these conflicts were also concerned with how the three Kingdoms of England, Scotland and Ireland should be governed. The outcome was threefold: the trial and the execution of Charles I (1649); the exile of his son, Charles II (1651); and the replacement of English monarchy with the Commonwealth of England, which from 1653 (as the Commonwealth of England, Scotland, and Ireland) unified the British Isles under the personal rule of Oliver Cromwell (1653–1658) and briefly his son Richard (1658–1659). In England, the monopoly of the Church of England on Christian worship was ended, and in Ireland, the victors consolidated the established Protestant Ascendancy. Constitutionally, the outcome of the wars established the precedent that an English monarch cannot govern without Parliament's consent, though the idea of Parliamentary sovereignty was legally established only as part of the Glorious Revolution in 1688.