The Second English Civil War was a war that should not have happened. The first had ended in 1646 with Charles I a prisoner; the Scots, who had fought alongside the English Parliament, had handed him over and gone home. But Charles kept negotiating, and in December 1647 he signed an agreement with a faction of the Scottish Parliament, the Engagers, who promised to invade England on his behalf in return for a guaranteed Presbyterian settlement. By the spring of 1648 the Engagers had pushed through the Scottish Parliament, repudiated the old treaty with the English, and seized Carlisle. Royalist mutinies broke out across England and Wales. Sir Thomas Fairfax was tied down at Colchester. Cromwell was finishing off the siege of Pembroke. The Scots came south with what looked like overwhelming force.

Cromwell took the surrender of Pembroke on 11 July and was on the road within a week. Rather than following the Scots up the western coast he marched east through Gloucester, Warwick and Leicester to put his army between Carlisle and London. By 12 August he had joined John Lambert's smaller northern force at Wetherby. The infantry had walked 287 miles in thirteen days. He had nine thousand men. The Royalists, somewhere ahead of him in Lancashire, had something like twice that. On 13 August he turned the combined army west across the Pennines, leaving his artillery behind so the column could move faster. By 17 August he was close enough to fight.

Hamilton refused to believe the Parliamentarians were where they were. Langdale, commanding the English Royalist contingent, kept sending warnings that Cromwell was bearing down on their east flank. Hamilton kept disregarding them. The historians Bull and Seed put it this way: such impertinence and impetuosity was hardly credible to a man with twice the numbers. Hamilton's main force was crossing the Ribble bridge south of Preston, strung out for miles. Langdale's 4,000 men, many ill-armed conscripts, were left to face the Parliamentarian advance alone on the high ground north-east of Preston. They were the wall standing between Cromwell and the largest part of the Royalist army.

The fight north-east of Preston lasted four hours. Langdale's men, dug into hedges and enclosed fields, held against the New Model Army's first assaults. The rain fell hard enough to soak matchlock muskets and the fighting collapsed quickly into close work with pike and musket butt. Wave after wave of Parliamentarian infantry came on. By the time Langdale's flanks gave way, the Royalists had been outflanked on both sides and the line broke. The pursuit pinned the survivors against the Ribble. Most of them were killed or captured. Hamilton's main Scottish force, now exposed, fought a second hard action for the Ribble bridge itself. The bridge fell to the Parliamentarians as night came on. Around a thousand Royalists were dead and four thousand were prisoners by the end of the day.

The surviving Royalists, still numerous, fled south through the night, abandoning their baggage, their ammunition train and their artillery; almost none of it was destroyed before daylight, and all of it was captured. Cromwell's cavalry chased them down the road. Two days later, at Winwick near Warrington, the exhausted Scottish infantry turned to fight again. They lost again. By 24 August Hamilton himself had been taken at Uttoxeter. The Second English Civil War was effectively over within a week of Preston. Charles I was beheaded the following January. England was a republic by May. Cromwell wrote afterwards that the victory was nothing but the hand of God. The historians of the field, considering the geography and the odds and the speed of the march that brought him there, have generally given him a little more of the credit.