When civil war broke out between King and Parliament in 1642, Gainsborough sat in an awkward position. The countryside around it supported Parliament. The town itself had Royalist sympathies. More importantly, Gainsborough commanded a vital crossing of the River Trent and stood on the roads leading north toward Hull and south toward Lincoln. Whoever held it controlled a piece of the strategic geography of the East Midlands. In March 1643 Sir John Henderson rode out from the great Royalist garrison at Newark and took the town for the King without resistance. The Earl of Kingston was put in command. From Gainsborough his cavalry harassed Parliamentary positions across Lincolnshire - raiding Louth and Market Rasen, intercepting gunpowder bound for Rotherham - until in mid-July Lord Willoughby of Parham launched a surprise night attack and took the town back. The Earl of Kingston was killed soon after, struck down by a round shot fired from his own Royalist side as he was being rowed downriver to imprisonment at Hull. A bad week to be Royalist.

The Royalists immediately sent Charles Cavendish - a kinsman of the Cavendishes of Welbeck, related to the future Duke of Newcastle - to retake the town. Parliament responded by gathering relieving forces. Sir John Meldrum brought men from Nottingham. Cromwell brought his Eastern Association horse from Cambridgeshire. The two columns met on 27 July at North Scarle, ten miles south of Gainsborough, and were joined by a detachment from Lincoln. They totalled around 1,200 men. The next morning, marching north, they brushed aside a Royalist advanced guard of a hundred horse at the village of Lea just south of Gainsborough, then pressed up the slope of Foxby Hill east of the town. There they found Cavendish's main force drawn up on the crest. The Parliamentarians attacked uphill. After fierce fighting the Royalists began to give way, then to flee, with Parliamentary cavalry in pursuit down the far side of the hill.

It is what happened next that earned the day its place in English history. Most of the Parliamentary horse, intoxicated with the chase, had gone after the retreating Royalists. Cromwell did not. Looking back across the field he saw Cavendish's reserve - a fresh regiment of horse - moving to cut off the Parliamentarians who had stayed behind. He gathered the three troops still under his hand and led them in a counter-charge against the reserve's flank and rear. The Royalists, unbalanced by the unexpected attack, were driven down the slope into the flat marshy ground that ran to the Trent. Three hundred died there. Cavendish was unhorsed and killed. The bog took his name, became Candish Bog in local memory, and stayed that way for centuries. Cromwell rode into Gainsborough with supplies for Willoughby's garrison, having lost very few men of his own.

The tactical victory did not save Gainsborough. While Cromwell was distributing his supplies, scouts brought news of a large force approaching from the north. Cromwell and Meldrum rode out with 600 of Willoughby's foot to investigate, drove off two troops of Royalist horse at Morton, climbed a hill and found themselves staring at the army of the Earl of Newcastle - perhaps thirty regiments of foot and a great body of horse - bearing down on the town to retake it for the King. The foot scattered in disorder. Cromwell, his own horses already exhausted by the battle and the chase, organised a fighting withdrawal. Two rearguard parties, one from his own regiment and one from Lincoln, stood firm in turn and retired alternately, covering each other's retreat in a textbook disciplined manoeuvre. They reached Lincoln having lost only two men. Newcastle besieged Gainsborough, and Willoughby surrendered after three days. The town went back to the King. But the Eastern Association cavalry had drawn the attention of the Parliamentary high command, and the man who had led that final charge on Foxby Hill - who knew when not to chase, and how to choose the moment - was on his way to Marston Moor, Naseby, and the Lord Protectorship.