Thus began one of the greatest cavalry charges in history. Somewhat obscured by the weather, Murat's squadrons charged through the Russian infantry around Eylau and then divided into two groups. The group on the right, Grouchy's dragoons, charged into the flank of the Russian cavalry attacking Saint-Hilaire's division and scattered them completely. Now led by Murat himself, the dragoons wheeled left against the Russian cavalry in the center and, joined by Hautpoult's cuirassier division, drove the Russian cavalry back on their infantry. Fresh Russian cavalry forced Murat and the dragoons to retire, but d'Hautpoult's cuirassiers burst through everything and the broken Russians were cut to pieces by fresh regiments of cuirassiers. D'Hautpoult then rode through the Russian guns, chasing off or sabering the gunners, and broke through the first line of Russian infantry, trampling a battalion that attempted to stand. The cuirassiers forced their way through the second line of Russians and only after 2,500 yards did the charge finally expend its force in front of the Russian reserves. A second wave of cavalry consisting of the Guards and Grouchy's dragoons now charged the Russians as they attempted to reform and also rode through both lines of infantry. Another group charged into the Russian infantry in the area where Augereau's corps had made its stand. Not content with these heavy blows, the cavalry reformed, wheeled and charged back again, finally retiring under the protection of the Guard cavalry. Murat had lost 1,000-1,500 well-trained troopers, but relieved the pressure on Augereau, Saint-Hilaire and Soult, paralyzing the Russians long enough to allow Davout to deploy in strength. Rarely had French cavalry played such a pivotal part in a battle; in part this was because, for the first time, Murat's men were now mounted on the best cavalry horses in Europe, freshly requisitioned in the aftermath of the conquest of Prussia.
Davout's corps, about 15,000 strong, was now in position and began to drive in the Russian left. Despite the disarray of the Russian center, Napoléon declined to follow up Murat's charge by advancing with the Guard. Such a move might have decisively won the battle, but Napoléon, well aware that 9,000 Prussians under L'Estocq and his chief of staff Gerhard von Scharnhorst were still unaccounted for, judged it wise to retain the Guard in reserve. Through the afternoon Soult, Augereau and Murat managed to hold their ground while Davout, assisted by Saint-Hilaire, gradually bent the Russian left back further and further, pushing it to a right angle with the Russian center. By 15:30 it seemed that the Russian cohesion would soon break, as their left was in full retreat.
For several crucial hours Bennigsen could not be found; he had personally ridden to L'Estocq to urge that general to hasten the march of his Prussian corps to the battlefield. His mission was successful; . L'Estocq's 9,000-man Prussian force, having lost a third of its strength to Ney's pursuit, approached the battlefield via the Russian right and passed completely behind the Russian position to its left wing, gathering strength in doing so by collecting Russian stragglers and adding them to the 6,000 Prussian troops. At 16:00 L'Estocq counterattacked by falling on Davout's exposed right flank, and the heartened Russians soon launched a fresh attack against Davout. Over the next three hours Davout was halted and forced back to a line running from the village of Kutschitten to near the village of Anklappen towards Saint-Hilaire's right by Eylau. Davout, alert to the danger, formed a battery of his guns on the heights of Klein Sausgarten and personally rallied his troops while his guns drove the Prussians back into the woods. With nightfall exhaustion set in and fighting on the Russian left petered out.
By then the roar of cannons on the Russian right announced Ney's arrival. Napoleon had not recalled Ney until 08:00 of the 8th when he realized that the Russians intended to fight. Although Ney was within marching distance of the battle, the heavy snow had muffled the sound of cannon fire and he was completely unaware of events until a messenger reached him around 10:30. Somewhat delayed by L'Estocq's rear guard, the leading division of Ney's corps did not reach the battlefield until around 19:00 and immediately swept forward into the Russian right and rear. Bennigsen counterattacked. Bitter fighting continued until 22:00, at which point both sides drew off a little. After a contentious council of war with several of his generals forcefully arguing for continuing the fight for a third day, at 23:00 Bennigsen decided to withdraw and, covered by the Cossacks, the Russians silently began to leave. The exhausted French did not even notice until 03:00 and were in no condition to pursue.