The War of the Fourth Coalition was fought between the First French Empire and a coalition of Prussia, Russia, Saxony, Sweden, and the United Kingdom from 1806 to 1807 amid the Napoleonic Wars.
Following Napoleon's victory at the Battle of Austerlitz in December 1805, the Austrian Empire signed an armistice with France. Napoleon focused on achieving victory over his two remaining antagonists, Britain and Russia, while isolating Prussia from their influence through alliance talks. Britain retained its dominance of the seas but was unable to check French advances on the mainland, while Russia spent most of 1806 recovering from defeats from the previous year's campaign. In July 1806, Czar Alexander I of Russia vetoed a tentative peace treaty with France. Meanwhile, Prussia grew angry at French transgressions such as Marshal Jean-Baptiste Bernadotte's movement through Prussian-controlled Ansbach to face the Austrians and Prussians and France's occupation of Hanover (bordering Prussia) since 1803. In March 1806, Napoleon made his brother-in-law Marshal Joachim Murat ruler of the Grand Duchy of Berg and the Duchy of Cleves, and he tactlessly ejected a Prussian garrison stationed in his realm. France also meddled in German affairs through the creation of the Confederation of the Rhine and the dissolution of the Holy Roman Empire, causing Prussia to view this French buffer state as a threat. While Napoleon initially intended to secure Prussia's alliance through the handover of Hanover, Prussia soon learned that Napoleon had secretly promised to return Hanover to Britain during his abortive peace negotiations, causing Prussia to reward France's duplicity with a declaration of war in August.
In September 1806, Napoleon deployed the corps of the Grande Armee along the frontier of southern Saxony, and he had his army march in three parallel columns through the Franconian Forest in southern thuringia to catch the Prussians unaware. On 9 October, the French brushed aside a Prussian division at the Battle of Schleiz, and the popular Prince Louis Ferdinand of Prussia was killed at the Battle of Saalfeld the following day and his division crushed. On 14 October, Napoleon smashed two Prussian armies at the Battle of Jena-Auerstedt, and Duke Ferdinand of Brunswick was killed. The French general Jean-Baptiste Bernadotte, who had failed to reach the battlefield in time, redeemed himself by mauling a Prussian reserve corps at the Battle of Halle on 17 October.
Soon, 160,000 French troops were engaged with Prussia, and reinforcements arrived across the Wesel bridgehead. By then, Prussia had lost 65,000 men, 150,000 captured, and two royal family members slain; the French had lost just 15,000 men. Napoleon entered Berlin on 27 October 1806 and visited the tomb of Frederick the Great, concluding a 19-day invasion of Prussia. Most of the shattered remnants of the Prussian army and the displaced royal family fled to Konigsberg, where they intended to link up with the approaching Russians and continue the fight. On 11 December 1806, Saxony defected to the Napoleonic alliance and joined the Confederation of the Rhine.
In late 1806, French forces entered Poland and Napoleon created a new Duchy of Warsaw, taking advantage of a Polish anti-conscription uprising against the Prussians. He then confronted the approaching Russian armies in the north, but he was defeated at the 26 December Battle of Pultusk while attempting to entrap and defeat Levin August von Bennigsen's Russian 1st Army. On 7-8 February, he fought the Russians to a draw at the Battle of Eylau, and the Russians were forced to withdraw further north. On 14 June, Napoleon won a final victory over the Russians at the Battle of Friedland, forcing Russia and Prussia to sue for peace at Tilsit on 7 July 1807.
Meanwhile, the Swedes defended Swedish Pomerania from French attack, defending the fort of Stralsund and pushing the French out of Pomerania in the 1-3 April 1807 Great Sortie of Stralsund. On 18 April, France and Sweden agreed to a ceasefire, but Sweden's refusal to join a blockade of Britain resulted in a second invasion and Stralsund's fall on 24 August. The French conquered Swedish Pomerania, and the Swedes withdrew with all their munitions of war.
Following the Treaties of Tilsit, only Britain and Sweden remained at war with France. Tilsit transformed Russia into a French ally, and Russia and Denmark-Norway waged naval wars against Britain. Russia also attempted to force Sweden to join the Continental System in the Finnish War of 1808-1809, conquering the Grand Duchy of Finland. Britain was left France's sole enemy until the outbreak of the Peninsular War in 1808.