destruction of its principal armies at Jena and Auerstedt. Most of the shattered remnants of the Prussian army and the displaced royal family escaped to refuge in Eastern Prussia near Königsberg, eventually to link up with the approaching Russians and continue the fight. Meanwhile, on 11 December 1806 the Treaty of Posen elevated Saxony to a kingdom upon allying with France and joining the Confederation of the Rhine, thereby leaving the Allied Coalition. On 21 November 1806, Napoleon issued the Berlin Decree to bring into effect the Continental System. This policy aimed to control the trade of all European countries without consulting their governments. The ostensible goal was to weaken the British economy by closing French-controlled territory to its trade, but British merchants smuggled in many goods and the Continental System was not a powerful weapon of economic war. Polish, Russian and Swedish campaigns Towards the end of 1806, the French entered Poland and Napoleon created a new Duchy of Warsaw, to be ruled by his new ally Frederick Augustus I of Saxony. The area of the duchy had already been liberated by a popular uprising that had escalated from anti-conscription rioting. Napoleon then turned north to confront the approaching Russian armies and to attempt to capture the temporary Prussian capital at Königsberg. In pursuit of this aim, twice his attempts to entrap and defeat Bennigsen's Russian 1st Army at Pultusk and in the vicinity of Heilsberg during the turn of the year were thwarted. A tactical and bloody draw at Eylau (7–8 February) forced the Russians to withdraw further north. After spending much of the spring recuperating his forces, Napoleon finally routed the Russian army at Friedland (14 June). Following this defeat, Alexander sued for peace with Napoleon at Tilsit (7 July 1807). Meanwhile, Swedish involvement was primarily concerned with protecting Swedish Pomerania. Despite being defeated at Lübeck, the Swedes successfully defended the fort of Stralsund and pushed the French forces out of Swedish Pomerania in early April, 1807. On 18 April, France and Sweden agreed to a ceasefire. However, Swedish refusal to join the Continental System led to a second invasion of Swedish Pomerania led by Marshal Brune. Stralsund fell on 24 August after a siege and the Swedish army abandoned Rügen, thus leaving France in control over Swedish Pomerania; the resulting armistice, agreed by Marshal Brune and Swedish general Johan Christopher Toll, had allowed the Swedish army