Gustav IV Adolf

Gustav IV Adolf of Sweden (1 November 1778 – 7 February 1837) was the son of Gustav III of Sweden and his queen consort Sophia Magdalena, eldest daughter of Frederick V of Denmark and his first wife Louise of Great Britain. He was the last Swedish ruler of Finland. Gustavia in Swedish Pomerania was named after him, but was lost in the Napoleonic Wars.

March 29, 1809 – King Gustav IV Adolf of Sweden abdicates after a coup d'état. At the Diet of Porvoo, Finland's four Estates pledge allegiance to Alexander I of Russia, commencing the secession of the Grand Duchy of Finland from Sweden.

Gustav IV Adolf arrested

Wikipedia:
Gustav IV Adolf's reign was ill-fated and was to end abruptly. In 1805, he joined the Third Coalition against Napoleon. His campaign went poorly and the French occupied Swedish Pomerania. When his ally, Russia, made peace and concluded an alliance with France at Tilsit in 1807, Sweden and Portugal were left as Great Britain's European allies. On 21 February 1808, Russia invaded Finland, which consisted of provinces of Sweden, on the pretext of compelling Sweden to join Napoleon's Continental System. Denmark likewise declared war on Sweden. In just few months after, almost all of Finland was lost to Russia.


http://www.histdoc.net/history/fr/frhamn.html

As a result of the war, on 17 September 1809, in the Treaty of Hamina, Sweden surrendered the eastern third of Sweden to Russia. The autonomous Grand Principality of Finland within Imperial Russia was established.