Right from the start, he clashed with Parliament. Prompted by family tied, Charles had promised aid to Frederick V, Elector Palatine, heavily engaged in the first years of the Thirty Years War,. who was married to Charles's sister Elizabeth, and had agreed to fund an English force which would join Frederick's general Ernst von Mansfeld. However, Charles's first Parliament, already hostile to his favourite, the duke of Buckingham, refused to fully fund this overseas venture. Involvement in the Thirty Years War continued, with a disastrous attempt against Cadiz in 1626, and another, equally disastrous attempt to help the Protestants of Rochelle in 1627, after which Charles made peace with both France (1629) and Spain (1630). The clashes with Parliament had continued, especially over religion, where Parliament was Calvinist, while Charles was suspiciously High Church, and was always suspected of pro-Catholic leaning. His third Parliament saw the signing of the 'petition of right' (June 1628), largely led by Sir Thomas Wentworth (soon to become earl of Strafford). Parliament was dissolved on 10 March 1629, beginning the Eleven Years Tyranny, during which Charles ruled without Parliament.
Charles now set about destroying his support. To fund his government, he revived a series of out of date sources of income, starting with tonnage and poundage in 1629, followed by fines for not taking up knighthood, and huge fines for encroaching of forest lands in 1630, and most famously, the demanding of Ship Money from 1634. At the same time, he was causing religious offence by his support for Archbishop Laud, a follower of the anti-Calvinist theologian Jacobus Arminius, who from 1633 was attempting to force the Puritanical party in the Church to accept church ceremonial.
It was his church policy that led to trouble with Scotland. At his Scottish coronation in 1633, he had caused offence with the level of ceremonial he required. In 1637, he attempted to impose a new liturgy on Scotland, drawn up by Laud, and further from Scottish practise than even the English liturgy of the period. In response, the Scots signed a National Covenant, reaffirming their opposition to the Catholic church, and to Charles's religious policies, and in November 1638 a general assembly abolished the bishops. Charles was furious, and raised an army, intending to invade Scotland (First Bishop's War, 1639), but he ran out of money, and after signing the Treaty of Berwick, decided to summon Parliament to raise funds for a renewed war. However, this Short Parliament (April-May 1640), led by John Pym, made it clear that they agreed with the Scots, and demanded to discuss the Scottish complaints first, at which point Charles dissolved it. The Scots now invaded England (Second Bishop's War), and captured Newcastle and Durham, forcing Charles to summon Parliament again.
The Long Parliament (November 1640-1660), was just as hostile as the Short had been. Within six months, it had achieved the execution of Wentworth, now earl of Strafford, and a key supporter of Charles, and had forced Charles to agreed not to dissolve Parliament without it's own agreement. However, for much of the rest of 1641, Parliament was deadlocked over Religion, and only a series of blunders by Charles led to war. In October 1641, rebellion had broken out in Ireland, and many in Parliament suspected Charles of at least prior knowledge. However, when he returned to London in November 1641, Charles was so well received that he decided to resist Parliament, culminating on 4 January 1642 with his attempt to arrest the 'five members' of the House of Commons, with united Parliament against him. On 10 January Charles left to collect troops, while Parliament moved to gain control of the militia. Finally, on 22 August 1642, Charles raised his standard at Nottingham, beginning the First Civil War.
From Nottingham, Charles moved west across the midlands, and into the Welsh marches, gathering an army, before on 12 October leaving Shrewsbury to march on London. Parliament sent Essex to stop him, and the two sides met in the battle of Edgehill (23 October), the first major battle of the war, and a draw. However, the result led to gloom in London, and Charles had a brief chance to capture his capitol. By the time he was ready to make that move, the mood of London had recovered, and Charles was faced by the trained bands of London at Turnham Green (13 November), after which he retreated to Oxford to spend the winter, concentrating on expanding the area he held around the city.
His next move was to besiege Gloucester, held for Parliament by Colonel Massey. Charles and his army arrived before the city on 10 August 1643, but the city held, and he was forced to abandon the siege on 8 September by the arrival of Essex. However, this gave Charles another chance for victory - if he could cut off Essex's retreat to London, and defeat him on a well chosen battlefield, London would have been exposed. However, Charles did not manage his march well, and when battle came (1st battle of Newbury, 20 September 1643), the result was inconclusive. However, after the battle Charles decided to march away to Oxford, leaving Essex free to return to London with his army intact.
Charles spend much of the next few months in Oxford, only leaving at the start of June 1644 when the city came under threat. After a short period of movement, Charles won a victory at Cropredy Bridge (29 June 1644), which much weakened Waller's army. Much of the gloss was taken off this victory by the defeat of Prince Rupert at Marston Moor (2 July), but one of Charles's failings during the civil war was an inability to understand the magnitude of some of his defeats. His response was to confirm an earlier intention to move against Essex, currently in the west country, where he had attempted to capture the Queen. The two armies came together around Lostwithel, and fought two battles, starting with Beacon Hill (21 August 1644), an ambitious attack on a four mile front by Charles, which achieved it's limited objectives and pined Essex and his army down. The second battle, Castle Dore (31 August-1 September 1644), which saw the surrender of 6,000 men, the remains of Essex's army, although Essex himself escaped. This Lostwithel campaign saw Charles I at his best as a commander, controlling a complex series of attacks that led to the defeat of one of Parliament's best armys. In many ways, it was Charles's high point during the war. Charles now moved slowly out of the west country, with the limited intention of relieving the sieges of Banbury Castle, Basing House and Donnington Castle. Parliament managed to bring together a large army, but at the 2nd battle of Newbury (27 October 1644), the divided command of the Parliamentary army allowed the King's much smaller army to escape, reaching Oxford on 1 November. His army was now 15,000 strong, and for the first time under the command of Prince Rupert. Moreover, Parliament now abandoned the three sieges in question.
The plan agreed on for 1645 was for Charles's army to march north to relief Chester, then attack the Scottish army, then besieging Pontefract. Charles marched on 9 May, and soon heard that the siege of Chester had been lifted. At the end of May, Prince Rupert captured Leicester, and Charles was in good spirits. That was soon to change. On 14 June, Charles's army came up against the New Model Army under Fairfax at the battle of Naseby, and was soundly defeated. Charles himself escaped the field, and managed to work his way to Hereford, where he began to raise another army. For the next few months he kept on the move, from Cardiff on 5 August, to Doncaster on 18 August, Huntingdon on 24 August, then back to Oxford, before returning to Hereford on 4 September. Meanwhile, news of Naseby had disheartened many besieged Royalist strongholds, and Charles's position was becoming increasingly dangerous. Even Prince Rupert found himself under suspicion, and after surrendering Bristol in September, Charles dismissed him from command, and ordered him to leave the country. Charles meanwhile moved once again to relieve Chester, besieged again, but on 24 September was forced to watch as his cavalry was defeated (Rowton Heath). Although Chester held on for another five months (until 3 February 1646), Charles decided to move on, and by mid-October was based at Newark. Even Charles could now see that the war was lost, and made renewed efforts to split his enemies diplomatically, sensing that there were growing divides between the Presbyterian Scots and the largely Independent New Model Army. It was this that prompted Charles into one last desperate gamble, and on 27 April 1646 he left his own camp at Oxford to surrender to the Scots, now besieging Newark.
Charles's hopes of turning the Scots into his allies did not last. Their demands for Presbyterianism to be imposed on England were unacceptable to him, and after they received a third of their back pay, the Scots handed Charles over to Parliament in January 1647. After nearly a year in confinement, on 11 November 1647, Charles managed to escape from imprisonment in Hampton Court, and make his way to Carisbrooke Castle on the Isle of Wight, where although he was held captive, he was free to negotiate with all sides. On 26 December he signed the 'Engagement' with the Scots, in which he agreed to impose Presbyterianism on England, in return for Scottish aid in regaining power. The ensuing Second Civil War saw all of Charles's remaining support defeated, and, worse for him, removed any last feeling of loyalty in Parliament, now convinced that they could never again trust Charles. Charles now dug his heals in, and refused to make any concession. Meanwhile, Pride's Purge left the army in command, and determined to be rid of Charles. After a brief, and deeply suspect trial, Charles I was executed in Whitehall on 30 January 1649, not the first English monarch to be deposed and killed, but the first to suffer that fate in public.