In the early years of the war Cromwell was not a major figure. He achieved an early success on 10 August 1642, when he prevented the Royalists from taking the plate from the Cambridge colleges, and was present at the battle of Edgehill with his small cavalry troop, but his role is unknown. In 1643 he was one of several Parliamentary commanders in East Anglia, and took part in a series of battles, now with a regiment of horse, soon to become the famous Ironsides. Cromwell and his associates spend most of the summer attempting to block the advance of the earl of Newcastle's northern army in their attempts to march south as part of Charles I's strategy for 1643. The first skirmish came on 13 May (battle of Grantham), where a parliamentary force led by Lord Willoughby of Parham, Sir John Hotham, and Cromwell defeated a 1,200 strong Royalist force under Charles Cavendish. The fighting then centred on Gainsborough, captured by Willoughby in July. Cromwell, ordered to go to his aid, once again encountered Cavendish (battle of Gainsborough, 28 July 1643), which saw the death of Cavendish. However, the Parliamentarians then found themselves faced by Newcastle's main army, marching to besiege Gainsborough, and rapidly retreated. Cromwell once again reduced to a junior role, as one regimental commander amongst many under the earl of Manchester. When battle came (Winceby, 11 October 1643), Cromwell's role was minor. He appears to have been taken out of the battle after his horse was killed, and missed most of the significant action, with the battle being won by Thomas Fairfax, who cavalry charge carried the day. Winceby helped decided Newcastle against marching towards London, and thus helped spoil Charles's plan for the year. Although Cromwell had not yet achieved his later prominence, he was well known for the quality of his cavalry, served as governor of the Isle of Ely, and was second-in-command to the earl of Manchester. When Parliament created the Committee of Both Kingdoms (16 February 1644), to control the war, Cromwell was one of the initial members. By Marston Moor (2 July 1644), he had enough to gain command of the Parliamentary left wing, and while his own role is perhaps unclear, his cavalry played the most important part in Parliament's victory, and for that Cromwell can take the full credit. He was still reletivly junior at the 2nd battle of Newbury (27 October 1644), only ranking as commander of the cavalry of the Eastern Association, in what was a very badly run battle for Parliament.
Only now does Cromwell begin to gain the importance we are used to seeing him with. In the aftermath of the failure at Newbury, Cromwell accused Newcastle of 'continued backwardness', and disputed his version of the battle. In the ensuing debate, it became clear that the army was in need of reform, and on 23 November 1644 Parliament ordered the Committee of Both Kingdoms to come up with a plan, and on 6 January 1645 the committee delivered the plan which was to produce the New Model Army. In the meantime, probably under Cromwell's direction, the Self Denying Ordinance was introduced in the House of Commons on 9 December. This would remove from command anyone who sat in Parliament, and was quickly passed by the Commons, although was refused by the Lords on 13 January 1645, and only passed by them on 3 April, in a version which made exception for Cromwell, who now became General of Horse, and second in command behind Fairfax.
By the time the Ordinance was picked, Cromwell was already back in action, having been sent to join Waller in the west country to deal with the threat posed by Goring, who despite some success in keeping Waller out of the west country was soon contained, allowing Cromwell to return for what was to be the deciding battle of the war. When the main armies came together at the battle of Naseby (14 June 1645), Cromwell commanded the Parliamentary right. As the battle developed, it was only on this flank that Parliament was initially victorious, but that was enough, and once Cromwell had driven off his direct opponents, and sent a force to stop them rallying, he was able to attack the now defenceless Royalist infantry, which was almost entirely destroyed, mostly through capture. The Royalist position now began to collapse. The New Model, including Cromwell, moved against Goring and the Royalist army in the west, which was effectively defeated at the battle of Langport (10 July 1645), where Cromwell's Ironsides played a key part in the Parliamentary victory. Cromwell then embarked on a series of sieges which rapidly captured Devizes castle (23 September), Winchester (6 October), and Basing House (14 October), before defeating some of the few remaining Royalist horse at Bovey Tracey (9 January 1646). By this point, the fighting was almost over, and for a short period, Cromwell was more concerned with politics than fighting.
The main issue facing the army was it's future. With the real fighting over, Parliament wanted to disband the army. Cromwell joined with Fairfax in forming the General Council of the Army, which over the summer of 1647 made it clear that it would not disband until satisfied both politically and financially. However, for the moment the spectre of civil war loomed again. After escaping to Carisbrooke, Charles I had made his 'engagement' with the Scots, and provoked the Second Civil War. By the summer of 1648 a Scottish army once again threatened the north, this time on behalf of Charles. Cromwell was sent north to deal with it, and was soon on the trail of the 17,000 strong Scottish army, with somewhere between 6,500 and 9,000 troops of his own. Despite being outnumbered, Cromwell's troops were in much better shape that the Scots, and, even worse, as Cromwell approached them at Preston, the Scots were badly spread out. The resulting battle of Preston (17-19 August) was Cromwell's first major battle as commander in chief, and he acquitted himself well, defeating a much larger force component by component, and ending the threat from the Scots.
Cromwell was now determined to see Charles removed for good. On 6 December, Pride's Purge removed those members of Parliament who might have objected to the trial and execution, and after a trial of doubtful legality, Charles was executed on 30 January 1649. While England began life without a monarch, in March 1649 Cromwell was given command in Ireland, where the Irish revolt still went on, by now under the earl of Ormonde, who had managed to unite most of Ireland against Parliament. Cromwell reached Dublin in mid August, taking Drogheda on 12 September, and Wexford on 16 October, in each case acting ruthlessly towards the garrison and any captured priests. However, Cromwell's time in Ireland was cut short by trouble in Scotland.
After an abortive landing for Charles II by Montrose, Charles, who had been in negotiation with the Scots, agreed on 23 June 1650 to accept the Covenant, thus gaining Scottish support. Charles landed in Scotland, and an army was raised. Parliament was not idle while this was going on. After Fairfax resigned command in protest at a plan to invade Scotland, Cromwell was appointed general, with command of the war against the Scots (26 June 1650). By 19 July, Cromwell, with 5,000 cavalry and 10,000 infantry of the New Model Army, was at Berwick. Against him was David Leslie, with an inferior army, recently recruited and lacking experienced officers. Cromwell crossed into Scotland on 22 July, secured the port of Dunbar, and moved against Leslie, defending Edinburgh. However, Cromwell found himself facing a strong defensive position, and was forced to withdraw back to Dunbar. His army was now suffering a very high level of sickness, and a second sortie north also ended on 1 September back in Dunbar. This time, Leslie cut the road links back to England, and Cromwell found himself trapped and relying on the sea for his supplies. Leslie now appears to have assumed that Cromwell was planning to retreat, and prepared to attack the beleaguered English army. However, it was Cromwell who took the initiative, and the resulting battle of Dunbar (3 September), saw the Scottish army almost wiped out, with 10,000 men taken prisoner. Dunbar was probably Cromwell's most impressive military victory, defeating an enemy who outnumbered him, with an army suffering from series sickness, and after a series of futile marches across hostile territory. However, the war was not over. Charles II was still in Scotland, and Leslie began to raise another army, this time including English Royalists, excluded until Dunbar, which gave it a more experienced core. After a period of manoeuvres in which Cromwell managed to cut the Royalist army off from it's supply lines, Charles II ordered his army to march south and strike for London. By 22 August 1651, Charles and Leslie had captured Worcester, where they once again faced Cromwell. The Royalist position was strong, but they only had 12,000 men to face Cromwell's 25,000-30,000. In the resulting battle of Worcester (3 September 1651), Cromwell was able to launch two separate attacks, separated by the river Severn, a plan that at one point threatened to backfire, before the Royalist's crumbled. Charles II was one of the few senior Royalists to escape, and the Civil Wars were finally over.
Cromwell now found himself the most important figure in the army, the true source of power in the country. On 20 April 1653 he dissolved the Rump of the Long Parliament, which was briefly replaced by the Barebones Parliament, itself desolved on 12 December 1653. Finally, on 16 December 1653 Cromwell accepted the post of Lord Protector, ruling at first with a single chamber Parliament, which Cromwell soon dispensed with. Amongst his first problems was the First Anglo-Dutch War (1652-4), which he soon managed to bring to an end. Cromwell proceeded to rule through the army, dividing the country up into twelve districts, each under the control of a Major-General, with a troop of cavalry to enforce their decisions. Finally, a new constitution was put in place 1657, which provided for a second chamber appointed by Cromwell, as well as giving him the power to appoint his own successor. Cromwell did not long survive the new arrangement, dying on 3 September 1658. He was buried in Westminster Abbey on 23 November, and for a brief period succeeded by his son Richard, but after the restoration his body was disinterred, and on 30 January 1661 hung on the gallows at Tyburn.