Charles was the first king to be raised within the Church of England. This religious divide made it impossible to marry the Infanta Maria of Spain, whom he visited incognito in 1623 with his friend George Villiers, who that year became, the duke of Buckingham. Instead he married the French princess Henrietta Maria, but only on condition that she was allowed the free practice of the Catholic religion and to control the upbringing of their children. Their marriage happened two months after Charles succeeded to the throne. At first the marriage was unhappy: Henrietta was only fifteen and did not seem schooled in the art of courtship. She disliked Charles's childhood friend, the duke of Buckingham, and may have felt there was more than male bonding between them. Charles clashed with the large retinue that Henrietta had brought from France, which included a bishop, twenty-nine priests and over four hundred attendants. Within a year he had despatched these back to France. In 1628, Buckingham was murdered and it seems, with both these barriers removed, that their relationship warmed, and their first child was born in May 1629 (but died the same day). Henrietta always managed to maintain a controlling hand over Charles and exerted an increasingly unwise influence.

For the first three years of his reign Charles was heavily influenced by Buckingham whose exploits, which earlier might have seemed all a joke, became politically dangerous. Thanks to Buckingham, Charles found himself at war, first with Spain and then (in 1627) with France, with the intention of aiding the Huguenots. Buckingham led abortive and costly expeditions in both campaigns, and caused further international scandal by allegedly seducing the queen of France. Charles was also anxious to assist his brother-in-law Frederick to regain the Rhine Palatinate. Parliament did not like Buckingham and refused to grant Charles the finances for the wars. Charles consequently took what other avenues he could to raise money, including drawing on his wife's dowry and exacting loans from the wealthier peers. He also failed to pay soldiers. This attitude and his cavalier approach to Parliament, which he only called when he chose and then tended to ignore, incensed the Commons. They drew up a Petition of Right in 1628 to control Charles’s excesses. Although he accepted it, he chose largely to ignore it. Charles's clash with Parliament continued beyond the murder of Buckingham in August 1628. In the end, when Charles adjourned Parliament in March 1629, he did not call another for eleven years, ruling in absolute authority and not seeking parliamentary sanction for his actions. He raised money through taxes and custom duties (known as "tonnage and poundage") as he chose, and also imposed forced loans and purchased knighthoods upon his wealthier subjects. Although he saved much expenditure by bringing the pointless wars to a close, he frequently did not pay members of the royal household, even though he continued to live in great luxury.

He further upset his subjects, particularly those in Scotland, when he attempted to bring the Scottish church in line with the Church of England, imposing a new service book on Scotland, and introducing his own prejudices within the church, especially a tendency - known as Arminianism - to oppose strict Calvinist views of predestination. This was associated with high church practices, which echoed much of the Catholic service. Charles worked with William Laud, the new archbishop of Canterbury, and Thomas Wentworth, later earl of Strafford, who, in 1628, was created President of the North. Between the three they endeavoured to create an absolutism in church and state with Charles as the "most absolute prince in Christendom", ruling by divine right.

In some actions Charles might almost be justified. Wentworth's activities in Ireland after 1633, where he was lord deputy, were remarkable compared to past campaigns. Whereas previously the expenditure on Ireland had always oustripped any revenues received, Wentworth made the island profitable through his imposition of taxes and custom dues, his elimination of piracy, the introduction of a sound agricultural programme relying on the cultivation of flax, and the transformation of the army into an orderly force. This was done with a heavy-handedness that did not endear Wentworth to the king's subjects, but it was effective.

Attempts to run roughshod over the Scottish church, however, did not work so well. The Scots rejected Laud's new service book and declared defiant loyalty to the old Kirk. They formed a National Covenant in 1638 in opposition to Charles's policies. Following Wentworth's suggestion, Charles decided to impose his intentions by force. He raised an army in the spring of 1639, only to discover how little loyalty he commanded from his troops. Many of the English defected, not expecting to be paid and regarding the Scots as oppressed. The planned force of 30,000 ended up as only 8,000. Charles was easily defeated by the determined Scots in what became known as the First Bishops' War. Charles was in a predicament. He did not have the finances for a second campaign and had no alternative but to summon Parliament in April 1639. This Parliament, known as "the Short Parliament" because it was dissolved after only a few weeks, refused to grant Charles money unless he heard their grievances. Charles refused. Again he raised an army and again the Scots defeated him in the Second Bishops' War that August. Charles was again forced to call Parliament, and this time he was not allowed to dissolve it (hence it became known as "the Long Parliament"). Parliament chose Wentworth as the scapegoat for Charles's rule of tyranny, treating the understanding that Wentworth had promised to summon the army from Ireland to subdue the Scots as evidence of treason. Charles was forced to sign Wentworth's death warrant, and he was executed on Tower Hill on 12 May 1641.

Charles conceded some of Parliament's wishes, particularly over the ship-taxes he had imposed, but he would not give way on his reform of the Church of England. Charles attempted to rally support in Scotland but without success. However, when he returned to London he believed some support amongst Parliament was swinging his way. When John Pym presented his list of grievances against the king, known as the Grand Remonstrance, in November 1641, it did not receive universal support. Charles, encouraged by his queen, believed he could swing the balance in Parliament by removing the main opposition. In January 1642 Charles entered the House of Commons with an armed guard intending to arrest the five primary offenders. The five Members, who included Pym, had already escaped. This was the final straw. There was much public opposition to Charles's actions, so he withdrew from London. Negotiations over the next seven months failed to reach any agreement with the king totally intractable and Parliament increasing its demands. Civil war became inevitable and the king formally declared hostilities at Nottingham on 22 August 1642.

This was not the first civil war to divide England, and it was not the first to result in the deposition of a king, but because it was the first and only war in England to result in the abolition of the kingship, it has become known as the Civil War. The Royalists, or Cavaliers, generally had the upper hand in the early encounters, though they failed to strike decisively at the first main engagement at Edgehill on 23 October 1642 against the Parliamentarians, or Roundheads, under Robert Devereux, third earl of Essex (and son of Elizabeth's favourite). As the conflict continued, so the Roundheads began to take the offensive, especially when Thomas Fairfax and Oliver Cromwell took command with their specially trained cavalry and restructured New Model Army. They achieved victories at Marston Moor in 1644 and particularly at Naseby on 14 June 1645. With his defeat Charles deliberated about his position for nearly a year before surrendering to the Scots, expecting greater clemency. For their part, the Scots expected Charles to meet their terms over his church reform, but when Charles defiantly refused, the Scots handed him over to the English. While Charles was held by the Scots at Newcastle, the English Parliament issued a set of terms which became known as the Propositions of Newcastle in July 1646. These terms were to agree to the Covenant, abolish episcopacy, authorise Parliament's control over foreign policy and the army and amend his reforms of the Church. Charles refused. When handed over to the English, Charles was confined to Hampton Court, where Fairfax and Cromwell sought to come to terms with him over a formal written constitution. Again Charles refused. Escaping from Hampton Court, he sought refuge on the Isle of Wight, where he was confined to Carisbrooke Castle. Charles now intriguingly played one party against the other, negotiating at once with both the Parliamentarians and the Scots. The Scots reached an agreement with Charles which became known as the Engagement, signed on 26 December 1647. Under its terms the Scots would restore Charles as their king provided he would accept Presbyterianism for a trial period. This would be imposed upon the English and the two kingdoms united - though in fact it was a Scottish takeover of the English Parliament. Although not all Scots were united over this agreement, it was sufficient for an army to invade England in July 1648, only to be decisively defeated by Cromwell in three engagements in August at Preston, Wigan and Warrington. Fairfax's army also rapidly subdued a Royalist revolt in southern England.

In January 1649 Charles was brought to trial for treason, on the grounds that he had fought against his subjects. Charles refused to recognize the court as having any authority over him and thus offered no defence. He remained dignified but disdainful of the proceedings. When the court delivered its verdict the 135 judges were split almost evenly, 68 finding him guilty and 67 innocent. Thus by a majority of just one, Charles was condemned to death. He was executed at Whitehall on 30 January. He wore two shirts so as not to shiver from the cold and give the impression he was afraid. The Scots were vehemently opposed to the execution of their monarch by the English parliament but following their recent defeat they felt powerless to react. They nevertheless transferred their allegiance to Charles's son, Charles (II), while in England the kingship was abolished.

Charles failed as a king in every respect except authority, and in that he presumed too much. He was an absolute dictator or autocrat who nevertheless, in practice, could not operate without the support of his Parliament. His dignity and defiance against the odds might make him a romantic figure were it not for his complete and utter intransigence and arrogance. Whilst he failed to pay his soldiers and supporters, he spent a small fortune on commissioning and acquiring works of art. Where Henry VIII and Elizabeth had the support of the people even though they acted in an almost similar way, Charles did not, because Charles worked against rather than for his subjects. It was left to his son to restore the humanity, if not the credibility, to the Crown.