The war of the Third Coalition ended with the Peace of Pressburg (today Bratislava) on December 26, 1805. During the course of the war there were talks between the Allies to enlarge their military power by adding Prussia to the coalition of Russia, Saxony, Sweden and the UK. A few members of the new coalition already were fighting within the Third Coalition. Finally, in 1806 Prussia joined the coalition because they feared the increasing military power of the French in Central Europe and the establishment of the French-controlled Confederation of the Rhine. Russia and Prussia massed their troops in Saxonia and mobilized for a new campaign.

On October 1, 1806, the Prussian king sent an ultimatum to Napoleon requesting that he withdraw his troops beyond the River Rhine, essentially declaring war on the French king. By that time Napoleon had six fully-equipped army corps stationed in Southern Germany on the soil of his new allies: Bavaria, Wuerttemberg and Baden. In response to Prussia's warning, the French Grande Armee invaded Prussian territory with a new fighting technique known as the "batallion-carre," which allowed Napoleon's forces a great deal of flexibility and defense on all flanks.

At the same time the Prussian army had a strength of approximately 130,000 men, but their fighting technique was one that was used by the Prussian Army of Frederic II in the 18th century. They were not trained to fight against Napoleon with his well-trained Grande Armee. The Prussian troops were supported by 20,000 men from Saxony-Weimar and Kurssaxony. Besides this, the army did not have many qualified generals. The Commander of the main army was the Duke of Brunswick, and the 2. army was led by Prince Louis Ferdnand. According to the Prussian plans agreed on September 25th in Naumburg, the attack against Napoleon should start in two columns on October 7th. This Information also reached Napoleon, and he couldn't believe that the Prussian King would attack a victorious army with almost no allies, because the Russian were still preparing their army and the UK had no land troops on German soil.

So Napoleon moved his troops close to the Prussian and Saxon border where the first action in the new war came at the Battle of Schleiz on October 9, 1806, a decisive French victory. The same happened on the following day, and Prince Louis Ferdnand was killed in action. On October 14, 1806, Napoleon smashed the Prussian corps at the double battle of Jena-Auerstedt, and Napoleon's Marshal Davout defeated with only one corps the main Prussian Army under the Command of the Duke of Brunswick, who was mortally wounded in the battle. On account of this, King Frederic Wilhelm III took over the command with no military experience leading an army. As result of the the two battles, the Prussian army showed signs of disintegration and retreated, followed by the French cavalry, and the Saxon troops returned home.

Halfway between Jena/Auerstedt and Berlin you still can find the City of Magdeburg presently with about 232,000 inhabitants. This city was until 1912 the strongest fortress in Prussia and you still can find today parts and buildings of the old fortress. The earliest defenses are dated back to the 13th century. In 1680 Madgeburg came to Brandenburg and in 1701 they started building new and improved fortifications and these works were completed in 1721. At this time the fortress covered an area of almost 500 acres, and the city itself only about 300. After 1740 no improvements were made, and since the end of the 18th century there has been no more repair work done. So it happened that when the Grande Armee approached the city, the fortress was handed over after a three week siege to the French troops. The Prussian garrison had a strength of 20,000 men.

On October 24th French troops reached Berlin and the winner of Auerstedt, Marshal Davout, had the honor to be the first to enter the city. In the course of the following weeks, almost the complete Prussian army surrender with only a few fortresses standing up. At the end of the year the French army had a strength of almost 200,000 men, now also supported by the Saxon army, and Prussia had only 25,000 men left. 25,000 soldiers were killed in the battle. Napoleon reached Berlin on October 27, 1806, and visited the tomb of Frederick the Great.

The rest of the Prussian army, including the Royal Prussian Family, escaped to East Prussia to link up with the approaching Russian troops. In the mean time, French troops continued to occupy more Prussian cities and fortresses. While in Berlin Napoleon issued a series of decrees on November 21st. The hardest one was the implementation of the Continental System, which tried to eliminate the power of the UK by closing the continent for British ships and trade.

On December 11, 1806, Saxonia became a kingdom and joined the Confederation of the Rhine. By the end of the year the new king got the newly established Duchy of Warsaw. During the winter the French troops suffered from the bad weather and cold, and the landscape was not the best for fast and rapid maneuvers. Food, ammunition, winter uniforms and all such things needed by soldiers were inadequate because of poor supply lines back to France. Inside the country there were no possibilities to get such things because the Russian and Prussian troops removed all supplies.

In the beginning of spring, Napoleon turned north to capture the city of Königberg and to confront the Russian and the Prussian armies. On June 14, 1807, they fought the Battle at Friedland. As result of the French victory, the Russian Tsar Alexander asked for peace for him and Prussia at Tilsit on July 7, 1807.

At same time while Napoleon's army fought the Russians, further French troops fought Swedish troops in Swedish Pommerania. At first the Swedish troops were successful, but in the final stage they had to surrender at Rügen by the end of August 1807. This cease fire and peace treaty allowed the Swedish army to withdraw with all their weapons and ammunition to Sweden, so the UK was the only one of all the Allies left.

With the Peace of Tilsit, Prussia was the great loser. Even the meeting of Queen Luise of Prussia with Napoleon did not bring any result. Prussia lost all territories west of the River Elbe, which were given to new kingdom of Westfalia whose king was a brother of Napoleon, King Jerome. The Polish territories came to the Duchy of Warsaw, which was raised to become a kingdom together with Saxonia. So Prussia lost approximately half of its country and had to pay harsh reparations. Furthermore, Prussia had to join the Continental System against the UK, the same goes for Sweden and Russia. After the Treaty of Tilsit, Napoleon and the French Empire hit their zenith, only Portugal was still outside of the French border and control.