At the same time, after an aborted attempt to relieve Taunton (Somerset), Parliament's Committee of Both Kingdoms had directed Fairfax to besiege Oxford, the King's wartime capital. Initially, Charles welcomed this move, as Fairfax would be unable to interfere with his move north. Then at the end of May he was told that Oxford was short of provisions and could not hold out long. To distract Fairfax, the Royalists stormed the Parliamentarian garrison at Leicester on May 31. Having done so, Prince Rupert and the King's council reversed their former decision and decided to march south to relieve Oxford. They sent messages for Goring to rejoin them, but that officer was reluctant to leave the West Country for several reasons.
Parliament had indeed been alarmed by the loss of Leicester, and Fairfax was now instructed to engage the King's main army. He accordingly marched north from Oxford on June 5. His leading detachments of horse clashed with Royalist outposts near Daventry on June 12, alerting the King to his presence. On June 13, the Royalists, who were now making for Newark (Nottinghamshire) so as to receive reinforcements, were at Market Harborough (Leicestershire).
Fairfax was eager to engage them, and held a council of war, during which Oliver Cromwell, recently re-appointed Lieutenant General, arrived with some cavalry reinforcements. The New Model Army moved in pursuit of the Royalist army, and late in the day Henry Ireton attacked a Royalist outpost at Naseby, six miles (10 km) to the south of the royalist army. The King now had to accept battle, or retreat with Fairfax at his heels. Early on June 14, urged on by Rupert, he took the former course.
Fairfax initially considered occupying the northern slopes of Naseby ridge. Cromwell believed that this position was too strong, and that the Royalists would refuse battle rather than attack it. He is said to have sent a message to Fairfax, saying, "I beseech you, withdraw to yonder hill, which may provoke the enemy to charge us". Fairfax agreed, and moved his army back slightly.
The Royalists did not see Fairfax's position until they reached the village of Clipston, just over a mile north of Naseby ridge. It was clearly impossible for the Royalists to withdraw without being attacked by the Parliamentarian cavalry while on the line of march and therefore at a disadvantage. Rupert therefore deployed the army to its right, where the ground appeared to be more favourable for cavalry.
The Parliamentarian army occupied a front about two miles long. They outflanked the Royalist left, but their own left flank rested like the Royalists' right flank on the Sulby Hedges. At the last minute, as the Royalists began to advance, Cromwell sent a regiment of dragoons under Colonel Okey into the Sulby Hedges, where they could fire into the flank of Rupert's cavalry.
On the Parliamentarian left, the opposing wings of horse paused briefly to dress ranks before charging into each other. Ireton's own regiment apparently repulsed their Royalist opposite numbers, but Ireton then led part of them to the aid of the beleaguered Parliamentarian infantry. His troopers were driven off by Royalist pikemen, and Ireton himself was unhorsed, wounded in the leg and face and taken prisoner. At the same time, the second line of Royalist cavalry broke most of the Parliamentarian horsemen. Some of Ireton's regiments, on the far left, were saved from destruction by the fire from Okey's dragoons, but the others broke and fled, some not stopping until they reached Northampton, away. The entire Royalist right wing had been committed to defeat Ireton, and none were left in reserve. Rupert either failed or was unable to rally the Cavalier horsemen, who galloped off the battlefield in pursuit of the fleeing Parliamentarians.
Meanwhile, the Parliamentarian right wing of horse under Oliver Cromwell and the Royalist Northern Horse faced each other, neither willing to charge to the aid of their infantry while the other could threaten their flank. Eventually after half an hour, the Royalist cavalry began to charge and Cromwell's troops moved to meet them. Langdale's men were not only outflanked and outnumbered two to one, but forced to charge up a slope broken up by bushes and a rabbit warren. After a brief contest they were routed. Unlike Rupert, Cromwell had roughly half of his wing uncommitted, as only the front line of Cromwell's wing had taken part in the defeat of Langdale. He sent only four divisions (roughly two regiments) after Langdale, and turned his reserves against the left flank and rear of the Royalist centre. Okey's dragoons mounted their horses and charged from their hedges against the Royalist right, as did some of Ireton's regiments which had partly rallied.
Some of the trapped Royalist infantry began to throw down their arms and call for quarter; others tried to conduct a fighting retreat. One regiment, apparently Prince Rupert's Bluecoats, stood their ground and repulsed all attacks. One eyewitness said "they [the bluecoats] were like a wall of brass". Eventually, Fairfax led his own Regiment of Foot and his Regiment and Lifeguard of horse against them from all sides. The Bluecoats' resistance was broken and Fairfax is said to have taken their standard in person. Archaeological evidence (recovered musket balls etc) suggests that this episode took place on the slopes of Castle Yard, a wooded eminence which once had a motte and bailey castle, about a behind the Royalist position at the start of the battle.
At some stage, the King attempted to lead his Lifeguard of Horse to the rescue of his centre or against Cromwell's troopers, but was prevented from doing so by the Scottish Earl of Carnwath, who seized his bridle, swore at him and said, "Would you go upon your death?". Seeing the King apparently swerve away from the enemy, his Lifeguard also retreated in disorder for several hundred yards.
Behind the Parliamentarian lines, Rupert's men had reached Naseby and the Parliamentarian baggage. The Parliamentarian camp guards refused to surrender, and Rupert eventually rallied his men and led them back to the battlefield. It was now too late to save the remnants of the Royalist infantry, and Rupert could not induce his men to make another charge. Fairfax halted and reorganised his lines, and when he resumed his advance, Rupert's cavalry rode off the field.
Fairfax's forces pursued Royalist survivors fleeing north towards Leicester. Many Royalists were butchered when they mistakenly followed what they thought was the main road to Leicester into the churchyard in the village of Marston Trussell, and were unable to escape their pursuers. Parliamentarian troops also hacked to death at least 100 women camp-followers in the apparent belief they were Irish, though they were probably Welsh whose language was mistaken for Irish.
Royalist military force had been shattered at Naseby. The King had lost his veteran infantry (including 500 officers), all his artillery, and many arms. He could never again raise an army of similar quality.
The Parliamentarians had also captured the King's personal baggage, with correspondence which showed he intended to seek support from the Irish Catholic Confederation, and Catholic nations in Europe. By publishing this correspondence, Parliament gained much support in favour of fighting the war to a finish. Within a year, the First Civil War ended in a Parliamentarian military victory.