The Fourth Coalition fought against Napoleon's French Empire and were defeated in a war spanning 1806–1807. 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 Hannover to Britain in exchange for peace. Prussia and Russia mobilized for a fresh campaign with Prussia 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. 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. He also established the Duchy of Warsaw, a Polish client state, ruled by his new ally the king of Saxony. At the end of the war Napoleon was 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. Hostilities on land resumed later in 1807, when a Franco-Spanish force invaded Britain's ally Portugal, beginning the Peninsular War. A further Fifth Coalition would be assembled when Austria