Sweden joined the Third Coalition against Napoleon in 1805, but the coalition fell apart after the peace at Tilsit in 1807, forcing Russia to become the ally of France. Russia invaded Finland in 1808 and forced Sweden to cede that province at the peace of Fredrikshamn in 1809. The inept government of King Gustav IV Adolf led to his deposition and banishment. A new constitution was introduced, and his uncle Charles XIII was enthroned. Since he was childless, Sweden chose as his successor the commander in chief of the Norwegian army, Prince Christian August of Augustenborg. However, his sudden death in 1810 forced the Swedes to look for another candidate, and once more they chose an enemy officer. Jean-Baptiste Bernadotte, Marshal of France, would be named the next king. Baron Karl Otto Mörner, an obscure member of the Diet, was the one who initially extended the offer of the Swedish crown to the young soldier. Bernadotte was originally one of Napoleon's eighteen Marshals.