Herbert had accompanied the army of Fairfax, and seeing the number and resolution of the troops, he hoped that a victory might be gained which would terminate for good and all this disastrous conflict.  The ground round Naseby is chiefly moorland.  The king’s army was drawn up a mile from Market Harborough.  Prince Rupert commanded the left wing, Sir Marmaduke Langdale the right, Lord Ashley the main body.  Fairfax commanded the center of the Roundheads, with General Skippon under him.  Cromwell commanded the right and Ireton the left.  Rupert had hurried on with his horse in advance, and coming upon the Roundheads, at once engaged them.  So sudden was the attack that neither party had formed its lines for battle, and the artillery was in the rear.  Between the armies lay a wide level known as Broadmoor.  It was across this that Rupert had ridden, and charging up the hill on the other side, fell upon the left wing of Fairfax.  Cromwell, upon the other hand, from the extreme right charged down the hill upon Langdale’s squadrons.  Prince Rupert, as usual, carried all before him.  Shouting his battle cry, “Queen Mary,” he fell upon Ireton’s left wing, and drove them from the field, chasing them back to Naseby, where, as usual, he lost time in capturing the enemy’s baggage.  Cromwell, with his Ironsides, upon the other hand, had broken Langdale’s horse and driven them from the field.  In the center the fight was hot.  The king’s foot had come up the hill and poured volley after volley into the parliament ranks.  Hand to hand the infantry were fighting, and gradually the Roundheads were giving way.  But now, as at Marston, Cromwell, keeping his Ironsides well in hand, returned from the defeat of Langdale’s horse, and fell upon the rear of the Royalists.  Fairfax rallied his men as he saw the horse coming up to his assistance.  Rupert’s troopers were far from the field, and a panic seizing the king’s reserve of horse, who had they charged might have won the day, the Earl of Carnewarth, taking hold of King Charles’ horse, forced him from the field, and the battle ended, with the complete defeat of the royal troops, before Rupert returned to the field of battle.

The Royalists lost in killed and prisoners five thousand men, their twelve guns, and all their baggage train, and what was of even greater importance, the king’s private cabinet, which contained documents which did more to precipitate his ruin even than the defeat of his army.  Here were found letters proving that while he had professed his desire to treat, he had no intention of giving way in the slightest degree.  Here were copies of letters to foreign princes asking for aid, and to the Papists in Ireland, promising all kinds of concessions if they would rise in his favor.  Not only did the publication of this correspondence and of the private letters between the king and queen add to the indignation of the Commons and to their determination to fight to the bitterest end, but it disgusted and alienated a vast number of Royalists who had hitherto believed in the king and trusted to his royal word.