In 1806, Napoleon had succeeded by a clever move in turning the Prussian government against him and luring it into the trap of an ill-considered war against him. For by the Treaty of Paris of 15 February 1806, Prussia was forced to occupy the Electorate of Hanover, which was bound to Britain by personal union, and at the same time to forbid the English the use of its ports. This prompted the British government to formally declare war on them. Napoleon, in turn, offered the British the return of the Electorate in secret negotiations, which apparently left the Prussians no other option but to declare war on the French Emperor on 8 October 1806 after this betrayal became known.
On 10 October, the first battle took place near Saalfeld in what is now Thuringia. The French, under Marshal Jean Lannes, were able to muster almost 23,000 soldiers, while on the Prussian side the Hohenlohe Corps with 19,000 Prussians and the Saxon allies with 20,000 men stood ready.
The previous day had already seen some insignificant skirmishing between the enemy armies, which ultimately led to the French being able to occupy the strategically important heights of the narrow Saale valley. Finally, 10 October began early in the morning with a cannonade lasting several hours from the fortified positions. In the course of the morning it became clear that the French (not least due to their battle-hardened troops and the advantage of their position) were superior to the allied Prussians and Saxons, and in the early afternoon Prince Louis Ferdinand ordered a retreat.
This was to turn into a complete debacle – the Prussian cavalry failed to cover the withdrawal of their army and when the gun batteries also became bogged down in a defile, the defeat of the Prussian army was sealed. For the French cavalry immediately knew how to take advantage of this and blew the now defenceless enemy apart. In the headlong flight, the entire Prussian artillery was lost and the allies suffered almost 2,000 dead and wounded, including Prince Louis Ferdinand, the third son of Prince Ferdinand of Prussia.
Although this battle was of no significance for the further course of the war, it was devastating for Prussian morale. For example, in the continuing panic, Saxon and Prussian troops fired on each other in the night of 10 to 11 October, each believing the other to be French, and only in the course of the next few days did the situation calm down enough for an orderly withdrawal to be considered.