By the mid-1640s, the British Isles had descended into total war. With the alliance between Parliament and the Scottish Covenanters, and the pact between King Charles and the Irish Confederate forces, soldiers from England, Ireland and Scotland fought each other across the three kingdoms. In Scotland, Charles's ally, the Earl of Montrose, led a brilliant guerilla campaign against the Covenanters - it became known as the 'Year of Miracles'.
In England, Oliver Cromwell emerged from this maelstrom as a great military genius. Though not the official commander of the Parliamentary army, as its Cavalry Commander, Cromwell won a string of remarkable victories. It was at Marston Moor in 1644 that Cromwell's leadership of men first became apparent. As weaker souls fled the field, Cromwell's charges, backed up by a reserve of Scottish cavalry, saved the day, and from Marston Moor onwards he proved to be an unbeatable tactician and an inspired organiser.
It was Cromwell's determination to win the war which led to the creation of the New Model Army - a professional army in which the troops were regularly paid, promoted on merit and properly trained. Above all, they were inspired by a religious zeal to beat the Cavaliers, a Christian army determined to cleanse the stain of popery from England. At the battle of Naseby, they demonstrated their ferocious power and utter commitment to the cause.
The rise of religious militants in the army frightened the Parliamentary old guard, the men who had first taken the initiative against the Crown in the early 1640s. As elements in the army became increasingly radicalised more conservative figures such as Essex and the leaders of the Scottish Covenanters became alarmed, fearing social anarchy and religious chaos. The growing tensions between the radical Independents in the army and the more moderate Presbyterians in Parliament and the Covenanter camp threatened to undermine the Roundhead-Covenanter alliance.
Charles sought to take advantage of these divisions for this own ends. Throughout the war, he was convinced that he would ultimately regain his monarchical powers- all he had to do was to exploit his opponents' divisions and make the best deal possible. Sensing the growing tensions within the Parliamentary side, in May 1646, Charles gave himself up to the Scottish Covenanters at Southwell in the hope of furthering these splits. It was a catastrophic decision, and he would never know freedom again.
After the uncertainties of 1642-43 and the Royalist advances of 1643, the war entered a crucial phase during 1644-45.
The Parliamentarians won an outstanding victory at Marston Moor in July 1644 which gave them control of the north of England. However, the Royalists continued to enjoy spectacular successes, especially in Scotland where Montrose enjoyed his 'year of miracles'. The second Battle of Newbury was inconclusive and the Parliamentary commanders seemed strangely unwilling to capitalise on their victories.
Parliament experienced major splits and divisions during 1644. Puritan Independents such as Cromwell and Ireton favoured vigorous prosecution of the war and a religious settlement based on liberty of conscience. The moderate Presbyterian 'old guard' favoured a negotiated end to hostilities and a restoration of their prestige and influence. By late 1644, the Independents had gained the upper hand, pushing through the Self-Denying Ordinance which severed the link between peers and MPs and military command (Cromwell was exempt) and, in early 1645, they followed this up by forming the New Model Army.
The conduct of the war during 1645 decisively influenced the long-term political settlement, and the news was universally disastrous for Royalism. Crushed at Naseby, vanquished at Bristol and slaughtered at Philiphaugh, by the end of the year, Royalism was dead as a military force.
After the destruction of his armies, Charles relied on his personality and tactics to gain advantage but he was no more gifted in this field than on the field of battle. After his deal with the Irish Confederates backfired disastrously, he surrendered to the Scottish Covenanters in May 1646 and brought the First Civil War to an end. However, as he haggled and negotiated with his hosts, violence erupted in Ireland once more.