Prussia had been maintaining a policy of neutrality since the treaty of Basil was concluded but Napoleon had his army pass through the Prussian territory at the time of the invasion of Hanover.
The King of Prussia, Frederick William III did not mind it, but the queen, Army officers and the foreign minister took it very seriously and pressurised the king to declare war.
Therefore, Frederick William put Napoleon on notice to vacate Naples, Germany, Switzerland and Piedmont and to return Hanover to Prussia which he had captured. But Napoleon did not pay any heed to this.
After the death of Pitt the Younger, the Prime Minister of England, Napoleon at the time of the treaty promised Hanover to England. It annoyed the Prussian Emperor, Frederick William, so much.
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He entered into a treaty with Russia and declared and started war against France before the coming of the Russian forces. Consequently, the Prussian forces were badly defeated at Jena and Averstadt on the same day.
Napoleon himself vanquished the Prussian commander Hohenlohe in the battle of Jena and French General Davout defeated the Prussian commander Brunswick in the battle of Averstadt.
Brunswick was killed in this battle but the other Prussian commander continued the battle, but at last he was also killed and the French army entered Berlin on 25th October 1806 and Prussian resistance collapsed completely.