Franco - Swedish War The Pomeranian War or the First War against Napoleon , was the first involvement by Sweden in the Napoleonic Wars when the country joined the Third Coalition to assist in defeating Napoleon . Background . In 1803 Britain had declared war on France , at this time Sweden had remained neutral together with the Nordic countries Denmark – Norway and Prussia . But after the execution of Louis - Antoine - Henri de Bourbon - Condé in 1804 , the Swedish government broke all diplomatic ties with France and concluded a convention allowing the British to use Swedish Pomerania as a military base against France , in exchange for payments . Russia also promised Sweden that 40 , 000 men would come to the aid of the country if it was threatened by French forces . So on 9 August 1805 Sweden joined the Third Coalition and declared war on France on 31 October . The war . The offensive against Hanover . In the beginning of November 1805 , a combined British , Russian and Swedish force of about 12 , 000 men were sent from Swedish Pomerania to liberate French - held Hanover . The offensive against Hanover was repeatedly delayed because of Prussia ' s partial reluctance that the Swedes and the Russians moved troops through Prussian territory . However , in December 1805 , after the battle of Austerlitz , the British and the Russian forces started to evacuate Hanover , leaving only a small Swedish force alone to face the French . In April 1806 , the Swedes were also forced to retreat back to Swedish Pomerania after an agreement had been concluded between Prussia and France . The Fourth Coalition . But during the summer of 1806 Prussia formed the Fourth Coalition against France , which gave Sweden the right to occupy Lauenburg . But during the autumn , the French forces advanced rapidly and soon much of the western German regions were occupied , this forced the Swedish troops on a retreat towards Lübeck . The plan was that the troops from there could take the sea route to Stralsund in order to avoid the advancing French forces . The Swedes were still caught by the French on the 6 November while they loaded their ships at Lübeck , and following the battle of Lübeck about 1 , 000 Swedish soldiers had to surrender to the numerically superior French forces . The French army began their offensive towards Swedish Pomerania in early 1807 and besieged Stralsund on 15 January . This was the beginning of a seven - month long siege , and since the French forces also were engaged in warfare elsewhere this increasingly reduced the number of troops stationed around Stralsund . When the Swedes were reinforced on 1 April it was decided that they would attempt to break the siege . This was done with some success since the Swedes managed to take Usedom and Wolin . But the French chose to counterattack , and a force of 13 , 000 men attacked the Swedes from Stettin on 16 April . This forced the left section of the Swedish army to withdraw , and another division in Ueckermünde was then cut off and later captured . On 18 April , France and Sweden agreed on a ceasefire according to which the French were to leave Pomerania . However , the Swedish government refused to join the Continental System and denounced the armistice under the influence of British diplomacy on 8 July . On 6 August 1807 , 50 , 000 French , Spanish and Dutch troops under Marshal Guillaume - Marie - Anne Brune began an assault on Swedish Pomerania and besieged Stralsund again . On 20 August 1807 , the defenders of the city capitulated and the remains of the Swedish army was surrounded at Rügen . However , Swedish General Johan Christopher Toll managed to concluded the convention of Schlatkow with Marshal Brune on favorable terms and his forces withdrew to Sweden with all of their munitions of war on 7 September . Franco - Russian treaty . The Franco - Russian Treaty of Tilsit left Britain and Sweden without other allies in the war against France . On 21 February 1808 , Russia joined the war against Sweden by invading Finland and on 14 March the same year , Denmark - Norway also declared war on Sweden . Danish and French - Spanish troops began preparations for an invasion of Skåne in Sweden , but the plan was soon aborted , and the war was instead directed to the Norwegian - Swedish border . Sir John Moore ' s expedition sent by the British government to protect Sweden from possible French - Danish attack arrived on 3 May 1808 and stayed until July when it was redirected to Portugal . Napoleon ' s plans to invade Sweden was never realized due to the British activity on the Baltic Sea , the weakness of the Danish military and hesitations of French Marshal Bernadotte . Bernadotte ' s actions made him popular enough to be elected as a Swedish Crown Prince after the coup d ' état in March 1809 . On 30 August 1809 , the new Swedish government was to conclude the Treaty of Fredrikshamn with Russia legitimizing the Russian annexation of Finland and Åland . A peace treaty between Sweden and Denmark - Norway was signed with no territorial adjustments on 10 December 1809 . Aftermath . On 6 January 1810 , Sweden signed a Russian - mediated Treaty of Paris with France regaining Pomerania , at a cost of joining the Continental System . On 17 November 1810 , Sweden was forced to declare war against Britain and all British goods in Swedish Pomerania were seized . The government - supported smuggling continued , however , over the North Sea and the English fleet was informed that it would be a phantom war . The war lasted until 1812 and no military action was taken .