The War of the Fourth Coalition (French: Guerre de la Quatrième Coalition) was a war spanning 1806–1807 that saw a multinational coalition fight against Napoleon's French Empire, subsequently being defeated. The main coalition partners were Prussia and Russia with Saxony, Sweden, and Great Britain also contributing. Excluding Prussia, some members of the coalition had previously been fighting France as part of the Third Coalition, and there was no intervening period of general peace. On 9 October 1806, Prussia declared war on France and joined a renewed coalition, fearing the rise in French power after the defeat of Austria and establishment of the French-sponsored Confederation of the Rhine in addition to having learned of French plans to cede Prussian-desired Hanover to Britain in exchange for peace. Prussia and Russia mobilized for a fresh campaign with France, massing troops in Saxony.
Napoleon decisively defeated the Prussians in an expeditious campaign that culminated at the Battle of Jena–Auerstedt on 14 October 1806. French forces under Napoleon occupied Prussia, pursued the remnants of the shattered Prussian Army, and captured Berlin. They then advanced all the way to East Prussia, Poland and the Russian frontier, where they fought an inconclusive battle against the Russians at the Battle of Eylau on 7–8 February 1807. Napoleon's advance on the Russian frontier was briefly checked during the spring as he revitalized his army with fresh supplies. Russian forces were finally crushed by the French at the Battle of Friedland on 14 June 1807, and three days later Russia asked for a truce. The war also saw the end of the British invasions of the River Plate where two unsuccessful British attempts to seize control of the Spanish colony of the Viceroyalty of the Río de la Plata, located around the River Plate in South America when the Spanish Empire was still an ally of Napoleonic France against Britain.
Through the Treaties of Tilsit in July 1807, France made peace with Russia, which agreed to join the Continental System. The treaty was particularly harsh on Prussia, however, as Napoleon demanded much of the Prussian territory along the lower Rhine west of the Elbe and in what was part of the former Polish–Lithuanian Commonwealth. Respectively, these acquisitions were incorporated into the new Kingdom of Westphalia, led by his brother Jérôme Bonaparte and into the new Duchy of Warsaw, a Polish client state, ruled by his new ally the king of Saxony. At the end of the war, there was peace on Continental Europe with Napoleon as master of almost all of western and central continental Europe, except for Spain, Portugal, Austria and several other smaller states.
Despite the end of the Fourth Coalition, Britain remained at war with France. War would return to Continental Europe later in 1807, when Napoleon decided to invade Portugal in order to compel Portugal to join the Continental System. A joint Franco-Spanish force invaded Britain's ally Portugal, beginning the Peninsular War where Napoleon would also invade Spain as well. A further Fifth Coalition would be assembled when Austria re-joined the conflict in 1809.
The Fourth Coalition (1806–1807) of Great Britain, Prussia, Russia, Saxony, and Sweden formed against France within months of the collapse of the previous coalition. Following his triumph at the Battle of Austerlitz and the subsequent demise of the Third Coalition, Napoleon looked forward to achieving a general peace in Europe, especially with his two main remaining antagonists, Britain and Russia. Meanwhile, he sought to isolate Prussia from the influence of these two powers by offering a tentative alliance, while also seeking to curb Prussia's political and military influence among the German states.
Despite the death of William Pitt in January 1806, Britain and the new Whig administration remained committed to checking the growing power of France. Peace overtures between the two nations early in the new year proved ineffectual due to the still unresolved issues that had led to the breakdown of the Peace of Amiens. One point of contention was the fate of Hanover, a German state in personal union with the British monarchy that had been occupied by France since 1803. Dispute over this state would eventually become a casus belli for both Britain and Prussia against France. This issue also dragged Sweden into the war, whose forces had been deployed there as part of the effort to liberate Hanover during the war of the previous coalition. The path to war seemed inevitable after French forces ejected the Swedish troops in April 1806.
Apart from some naval clashes and the peripheral Battle of Maida in southern Italy in July 1806 (though these actions are considered part of the tail end of the War of the Third Coalition), the main conflicts between Britain and France during the Fourth Coalition would involve no direct general military confrontation. Rather, there was an escalation in the ongoing economic warfare between the two powers. With Britain still retaining its dominance of the seas, Napoleon looked to break this dominance (after his defeat of Prussia) with his issuance of the Berlin Decree and the beginnings of his Continental System. Britain retaliated with its Orders in Council several months later.
In the meantime, Russia spent most of 1806 recovering from defeats from the previous year's campaign. Napoleon had hoped to establish peace with Russia and a tentative peace treaty was signed in July 1806, but this was vetoed by Tsar Alexander I and the two powers remained at war. Though nominally an ally in the coalition, Russia remained a dormant entity for much of the year (giving virtually no military aid to Prussia in the main battles that October, as Russian armies were still mobilising). Russian forces would not fully come into play in the war until late 1806 when Napoleon entered Poland.
Finally, Prussia had remained at peace with France the previous year, though it did come close to joining the Allies in the Third Coalition. A French corps led by Marshal Bernadotte had illegally violated the neutrality of Ansbach in Prussian territory on their march to face the Austrians and Russians. Anger by Prussia at this trespass was quickly tempered by the results of Austerlitz, and a convention of continued peace with France was signed two weeks after that battle at Schönbrunn. This convention was modified in a formal treaty two months later, with one clause in effect promising to give Hanover to Prussia in exchange for Ansbach's being awarded to France's ally Bavaria. In addition, on 15 March 1806 Napoleon elevated his brother-in-law Marshal Joachim Murat to become ruler of the Grand Duchy of Berg and Duchy of Cleves (acquired from Bavaria in return for its receiving Ansbach). Murat exacerbated Prussian enmity by tactlessly ejecting a Prussian garrison that was stationed in his newly acquired realm, prompting a stern rebuke from Napoleon. Relations between France and Prussia quickly soured when Prussia eventually discovered that Napoleon had secretly promised to return sovereignty of Hanover back to Britain during his abortive peace negotiations with the British. This duplicity by the French would be one of the main causes for Prussia declaring war that autumn.
Another cause was Napoleon's formation in July 1806 of the Confederation of the Rhine out of the various German states which constituted the Rhineland and other parts of western Germany. A virtual satellite of the French Empire with Napoleon as its "Protector", the Confederation was intended to act as a buffer state from any future aggressions from Austria, Russia or Prussia against France (a policy that was an heir of the French revolutionary doctrine of maintaining France's "natural frontiers"). The formation of the Confederation was the final nail in the coffin of the moribund Holy Roman Empire and subsequently its last Habsburg emperor, Francis II, formally abolished the empire. Napoleon consolidated the various smaller states of the former Holy Roman Empire which had allied with France into larger electorates, duchies and kingdoms to make the governance of non-Prussian and Austrian Germany more efficient. He also elevated the electors of the two largest Confederation states, his allies Württemberg and Bavaria, to the status of kings.