At Tilsit , Napoleon and Alexander had agreed that Russia should force Sweden to join the Continental System , which led to a Russian invasion of Finland in February 1808 , followed by a Danish declaration of war in March . Napoleon also sent an auxiliary corps , consisting of troops from France , Spain and the Netherlands , led by Marshal Jean-Baptiste Bernadotte , to Denmark to participate in the invasion of Sweden . But British naval superiority prevented the armies from crossing the Øresund strait , and the war came mainly to be fought along the Swedish-Norwegian border . At the Congress of Erfurt ( September – October 1808 ) , France and Russia further agreed on the division of Sweden into two parts separated by the Gulf of Bothnia , where the eastern part became the Russian Grand Duchy of Finland . British voluntary attempts to assist Sweden with humanitarian aid remained limited and did not prevent Sweden from adopting a more Napoleon-friendly policy
Congress of Erfurt
Congress of Erfurt
Napoleonic Wars