From the start of his reign, Charles I had been in conflict with Parliament. His believe in the divine right of kings meant that he was bound to oppose any attempt by Parliament to restrict his authority, and he dissolved his first Parliament in 1626 because they had demanded limits on Charles' right to levy customs, and a second in 1629 after protests over taxation, the war with Spain, and attempts to allow toleration for Catholics. From 1629 until 1640, Charles ruled without Parliament, a policy that worked as long as Charles did not need large amounts of money, which only Parliament could grant him. The inevitable crisis was caused by Charles's attempts to impose the English liturgy in Scotland. Defeat in the First and Second Bishop's Wars, forced Charles to call first the Short Parliament (1640) and then the Long Parliament (1640-1660) in order to raise the funds. Under serious pressure, Charles was forced to make concessions including to allow the execution of Lord Stratford, one of his key allies, and the Triennial Act, with made provision for Parliament to assemble every three years without needing the King's summons. The situation was made worse by the outbreak of rebellion in Ireland in October 1641, where fighting continued until Cromwell's infamous campaigns in 1649-50. This intensified the conflict between the King and his Parliament, who feared the consequences if Charles was allowed control of any army raised to suppress the Irish rebellion. On 1 December 1641, John Pym and John Hampden, leaders of the Puritan elements in Parliament, managed to pass the Grand Remonstration, a list of grievances against Charles, who responded with a bungled attempt to arrest in person five leaders of the commons on 3 January 1642 (To this day, the monarch of the day is not allowed to enter the Commons chamber because of this action). Over the next few months, both sides prepared for war. Charles established a base at York (March), and in April suffered his first rebuff. On 22 April, he sent his son to Hull, where the Governor, Sir John Hotham, had agreed to hold the city, and more importantly, the magazine, for Parliament. The following day, Charles himself rode to Hull, and demanded entrance. Hotham appeared on the walls, and denied Charles entry to the town, a moment of defiance that for many marks the start of the war.On 27 May, Parliament declared that the King was making illegal war on them, and claimed sovereignty, and in July started to raise an army under Robert Devereux, earl of Essex. The same month saw the first fighting of the war. Charles made a move against Hull, under the impression that Hotham had agreed to surrender the city if any attack was made. However, Parliament had suspected Hotham, and sent a professional soldier to aid him, and Charles was driven off. At the same time, Royalist and Parliamentary forces had clashed in Manchester, where on 15 July 1642 the Royalists under Lord Strange were forced out of the city. Finally, on 22 August 1642, Charles raised his standard at Nottingham, and declared Parliament to be traitors.
Although the civil war split all levels of society, some generalisations about the support for each side can be made. On Charles's side were the aristocracy, the peasants, the Anglican church, catholics; the north and the west. On Parliament's were the new commercial classes, the navy, the puritans; the south, the midlands and London. From the start, Parliament had the larger and better supplied army. Neither side had many experienced troops,. although both had experienced officers, many of whom had fought on the continent, where the Thirty Years War was slowly dragging towards it conclusion.

The first months of the war saw Charles's best chance of victory. With 11,000 men, Charles marched on London, to meet the Parliamentary army of 13,000 men under the earl of Essex at the battle of Edgehill (23 October 1642). The battle was indecisive, in part due to indiscipline on the part of the royalist cavalry under Prince Rupert, which took too long to return to the field after an initial charge, although Essex was forced to retreat on London. For a moment, London lay before Charles but he missed his chance, and when in early November he marched towards the capital, he faced Essex, now with 20,000 men, after reinforcement by the militia. The armies met at the battle of Turnham Green (13 November 1642). Charles retreated after what was in effect a brief skirmish, recognising that he could not win against such odds. Much of the fighting that followed was of little importance. On 25 September 1643, Parliament agreed to the Solemn League and Covenant, which gained them Scottish support by agreement to impost Presbyterianism across Britain. As a result, Scottish troops entered northern England in 1644, ending Royalist dominance in Yorkshire, weakening Charles. From April to June, Parliamentary forces were able to besiege York, only giving up when Prince Rupert arrived on the scene. The two armies met at the battle of Marston Moor (2 July 1644). The Parliamentary army, jointly commanded by Manchester, Leven and Fairfax, numbered 27,000, to Prince Ruperts 18,000. The Royalist cavalry once again began the battle with success, but Oliver Cromwell's Ironsides held their discipline, and defeated the royalist cavalry, before helping to destroy the royalist centre. Royalist loses were 3,000, Parliamentary loses 2,000, but Parliament could better afford the lose. The battle ended Charles's power in the north, and was followed by the surrender of royalist strongholds such as York and Newcastle. Despite this, Charles continued to be successful in the south in the autumn of 1644, and the year ended with Charles secure in Oxford.

At this point, Oliver Cromwell starts to come to the fore. For some time he had been proposing wholescale reform of the army, and Finally, in February 1645 his plans were approved. The new force, of 22,000 men, was to be raised by impressment, and funded by regular taxation, making it a proper standing army. This New Model Army was the first English army to wear a red uniform, and was by far the best organised army of the war. It solved many problems facing Parliament, not least of which was the unwillingness of the militias to fight away from their homes. Despite creating the new army, command of it was given to Fairfax, who replaced Essex, rather than to Cromwell.

The value of the New Model Army was soon seen. The battle of Naseby (14 June 1645), saw the start of the royalist collapse, and the beginning of the rise of Cromwell. Charles was outnumbered by nearly two to one, but once again Prince Rupert's cavalry defeated their royalist opponents, before once again chasing them off the battlefield. In contrast, on the other wing where Cromwell's cavalry had defeated their opponents, Cromwell's disciplined troops remained in the battle, and smashed the Royalist centre. When Rupert's troops finally returned to the battle, they refused to attack Cromwell's Ironsides. Charles was forced to flee to Leicester, while Cromwell's key role in the victory saw his influence vastly increased.

After Naseby, Charles's cause collapsed. Stronghold after stronghold surrendered to the Parliamentary forces, and finally Charles surrendered to the Scots (5 May 1646), partly in preference to surrender to Parliament, and partly because he seems to have believed he could regain Scottish loyalty, but was eventually handed over to Parliament (January 1647). A brief Second Civil War followed in 1648, but also saw Parliament overthrown by it's own army, after Parliament started negotiations with Charles. On 30 January 1649, Charles I was beheaded, after which the monarchy was abolished, and a Commonwealth established, in effect a military dictatorship, fronted by Cromwell.