The twin battles of Jena and Auerstädt were fought on October 14, 1806 near Naumburg in today's Germany, between the forces of Napoleon I of France and Frederick William III of Prussia.

Both armies were split into separate parts: Napoleon's main force at Jena consisted of Soult's IV Corps, Lannes' V Corps, Ney's VI Corps, Augereau's VII Corps, and the cavilry of Murat, about 96,000 men in total. Further north, in the vicinity of Auerstädt, was Bernadotte's I Corps (20,000 strong) and Davout's III Corps (27,000). Opposing them, the Prussian king had three forces: 55,000 men under the Duke of Brunswick, 38,000 under Prince Hohenlohe, and 15,000 under General von Ruchel .

The battles began when elements of Napoleon's main force encountered Hohenlohe's troops near Jena. Initially only 48,000 strong, the Emperor 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 Auerstädt , both Davout and Bernadotte received orders to come to Napoleon's aid. Davout attempted to comply, but Bernadotte, for reasons never fully explained, did not. Davout's route south, however, was blocked by the Prussian main force of 55,000 men, including the King in person. 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.

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". 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 Auerstädt and within marching distance of Jena, he ignored his orders and did not fire a shot in either battle. Davout was made Duke of Auerstädt. Soult, the hero of Jena, was not so honoured, possibly because Napoleon judged it best for reasons of prestige to keep the glory for himself.

On the Prussian side, Brunswick had been mortally wounded at Auerstädt, and over the next few days the remaining forces were unable to mount any serious resistance to Murat's ruthless cavalry pursuit. Davout led his exhausted III Corps into Berlin on October 25th. Hohenlohe's force surrendered on October 28th, Blucher's on November 7th. Isolated Prussian resistance remained, but Napoleon's primary foe was now Russia, and the Battle of Eylau awaited.

See also

• Battle of Jena
• Battle of Auerstädt