The twin battles of Jena and Auerstedt (older name: Auerstädt) were fought on 14 October 1806 on the plateau west of the river Saale in today's Germany, between the forces of Napoleon I of France and Frederick William III of Prussia. 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.
Several figures integral to the reformation of the Prussian Army participated at Jena–Auerstedt, including Gebhard von Blücher, Carl von Clausewitz, August Neidhardt von Gneisenau, Gerhard von Scharnhorst, and Hermann von Boyen.
Both armies were split into separate parts. The Prussian Army was in a very poor state. Brunswick was 71 years old while his field commanders were in their 60s. The Prussian army was still using tactics and training from the time of Frederick the Great. Its greatest weakness was its staff organization. Most of the divisions were poorly organized and did not communicate well with each other. The Prussians had three forces:
The Grand Armée loved their Emperor and their generals. The army was very experienced and was very well led, with a good mix of older, more experienced Marshals, and younger, upcoming Marshals. Napoleon's main force at Jena consisted of about 96,000 men in total: