In August 1806, the Prussian king, Friedrich Wilhelm III decided to go to war independently of any other great power except the distant Russia.
The Russian army, an ally of Prussia, was still far away when Prussia declared war.
In September, Napoleon unleashed all the French forces east of the Rhine.
Napoleon himself defeated a Prussian army at Jena (14 October 1806), and Davout defeated another at Auerstädt on the same day.
Some 160,000 French soldiers (increasing in number as the campaign went on) attacked Prussia, moving with such speed that they destroyed as an effective military force the entire Prussian army of 250,000—which sustained 25,000 casualties, lost a further 150,000 prisoners and 4,000 artillery pieces, and over 100,000 muskets stockpiled in Berlin.
At Jena, Napoleon fought only a detachment of the Prussian force. Auerstädt involved a single French corps defeating the bulk of the Prussian army.
Napoleon entered Berlin on 27 October 1806. He visited the tomb of Frederick the Great and instructed his marshals to remove their hats there, saying, “If he were alive we wouldn’t be here today”.
In total Napoleon had taken only 19 days from beginning his attack on Prussia until knocking it out of the war with the capture of Berlin and the destruction of its principal armies at Jena and Auerstädt.
By contrast, Prussia had fought for three years in the War of the First Coalition with little achievement.
http://en.wikipedia.org/wiki/Napoleonic_Wars
The Germans only truly became a world power when they united under Chancellor Bismark. Before that, the most powerful of the German principalities was indeed Prussia, but Prussia was still not able to successfully conquer the other great European nations alone. They had to always rely on their many treaties with other countries such as Austria and Russia.
The funny thing about Napolean, time and again he was able to beat the Austrians, Prussians and Russians in battle. Yet the only two times he personally faced the English (Egypt and in Belgium) he went down to humiliating defeat. The Iberian Peninsular wars were of such minor consequence to him, he never got personally involved.