The Finnish War was fought between Sweden and the Russian Empire from February 1808 to September 1809. As a result of the war, the eastern third of Sweden was established as the autonomous Grand Duchy of Finland within the Russian Empire. Other notable effects were the Swedish parliament's adoption of a new constitution and the establishment of the House of Bernadotte, the new Swedish royal house, in 1818.

After the Russian Emperor, Alexander I concluded the 1807 Treaty of Tilsit with Napoleon, he suggested that the Swedish King Gustav IV Adolf should join the Continental System. The king, who viewed Napoleon as the Antichrist, Britain as his ally against Napoleon's France and was apprehensive of the system's ruinous consequences for Sweden's maritime commerce, instead entered into negotiations with Britain in order to prepare a joint attack against Denmark, whose Norwegian possessions he coveted.

In the meantime, the Royal Navy attacked Copenhagen and the Anglo-Russian War was declared. Referring to the treaties of 1780 and 1800, the emperor demanded that Gustav Adolf close the Baltic Sea to all foreign warships. Although he reiterated his demand on November 16, 1807, it took two months before the king responded that it was impossible to honour the previous arrangements as long as the French were in control of the major Baltic ports.