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Which groups fought against Charles I during the English Civil Wars?

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Painting of the Battle of Naseby shows cavalry and infantry clashing, with soldiers on horseback leading a charge and fallen fighters on the ground amid the chaos of battle. — English Civil Wars *Battle of Naseby*, by an unknown artist. The victory of the Parliamentarian New Model Army, under Sir Thomas Fairfax and Oliver Cromwell, over the Royalist army, commanded by Prince Rupert, at the Battle of Naseby (June 14, 1645) marked the decisive turning point in the English Civil War.

During the English Civil Wars, King Charles I and his Royalist forces (also known as the Cavaliers) were opposed by the Parliamentarians, who were commonly referred to as the Roundheads.1

The Parliamentarian effort was bolstered by a critical alliance with the Scottish Covenanters. Under the Solemn League and Covenant signed in 1643, the Scots agreed to provide military support to the English Parliament in exchange for the establishment of Presbyterianism in England.12

To improve their military effectiveness, Parliament eventually centralized its forces by creating the New Model Army in 1645. Led by Sir Thomas Fairfax and Oliver Cromwell, this professional standing army became the decisive force that defeated the Royalists and ultimately exerted significant political influence over the kingdom.134