The twin battles of Jena and Auerstedt were battles between the forces of Napoleon I of France and Frederick William III of Prussia on the plateau west of the river Saale in today's Germany, on 14 October 1806. The decisive defeat suffered by the Prussian Army subjugated the Kingdom of Prussia to the French Empire until the Sixth Coalition was formed in 1812.

Initially only 48,000 men strong, Napoleon took advantage of his carefully planned and flexible dispositions to rapidly build up a crushing superiority. The Prussians were slow to grasp the situation, and slower still to react. Before Ruchel's 15,000 men could arrive from Weimar, Hohenlohe's force was routed. Nevertheless, it was a fierce battle, and Napoleon mistakenly believed that he had faced the main body of the Prussian army.

Further north at Auerstedt, both Davout and Bernadotte received orders to come to Napoleon's aid. Davout attempted to comply via Ekartsberg; Bernadotte, via Dornburg. Davout's route south, however, was blocked by the Prussian main force of 55,000 men, including the Prussian King, the Duke of Brunswick and Field Marshals von Möllendorf and von Kalckreuth. A savage battle ensued. Although outnumbered two to one, Davout's superbly trained and disciplined III Corps endured repeated attacks before eventually taking the offensive and putting the Prussians to flight. Though in sight of the battle, Bernadotte took no steps to come to Davout's aid, for which he was later censured by Napoleon.

Brunswick ordered his infantry to the left flank and his cavalry to the right. The rest of the French cavalry arrived at 9 a.m. and was placed on Gudin's left. General Louis Friant's Division and the 12-pound artillery arrived at 9:30 a.m. and moved in squares on Gudin's right. The advance of the French squares forced Blucher's cavalry back. Seeing no other option available he ordered his cavalry to attack. At this very moment two of Wartensleben's regiments attacked Hassenhausen.

Oswald's infantry and the Prince of Orange, the later William I of the Netherlands, arrived about 10:30 a.m., and the King made his only decision of the day, to split Orange's command in two, half to each flank. On the French side, Morand's Division arrived and was sent to secure Gudin's left. Davout could now see that the Prussians were wavering, so at 11 a.m. he ordered his infantry to counter-attack. By noon Schmettau's centre was broken and forced back over the Lissbach Stream, Blucher's cavalry was blown, and Wartensleben was trying to reposition his troops. The Prussians realised all was now lost and the King ordered a withdrawal.

Napoleon initially did not believe that Davout's single Corps had defeated the Prussian main body unaided, and responded to the first report by saying "Tell your Marshal he is seeing double", a reference to Davout's poor eyesight. As matters became clearer, however, the Emperor was unstinting in his praise. Bernadotte was severely censured and came within an ace of being dismissed on the spot — despite being within earshot of Auerstedt and within marching distance of Jena, he ignored his orders and did not participate in either battle. Davout was made Duke of Auerstedt. Lannes, the hero of Jena, was not so honored, possibly because Napoleon judged it best for reasons of prestige to keep the glory for himself.

On the Prussian side, Brunswick was mortally wounded at Auerstedt, and over the next few days the remaining forces were unable to mount any serious resistance to Murat's ruthless cavalry pursuit. In the Capitulation of Erfurt on 16 October, a large body of Prussian troops became prisoners with hardly a shot being fired. Bernadotte crushed Eugene Frederick Henry, Duke of Württemberg's Prussian Reserve on the 17th in the Battle of Halle.