During the Napoleonic Wars until 1810, Sweden and Great Britain were allies in the war against Napoleon. As a result of Sweden's defeat in the Finnish War and the Pomeranian War, and the following Treaty of Fredrikshamn and Treaty of Paris, Sweden declared war on Great Britain. The bloodless war, however, existed only on paper, and Britain was still not hindered in stationing ships at the Swedish island of Hanö and trade with the Baltic states.

The Finnish War was fought between the Kingdom of Sweden and the Russian Empire from 21 February 1808 to 17 September 1809 as part of the Napoleonic Wars. As a result of the war, the eastern third of Sweden was established as the autonomous Grand Duchy of Finland, a state under the rule of 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.