An extended Southwest Finland was made a titular grand duchy in 1581, when King John III of Sweden, who as a prince had been the Duke of Finland (1556–1561/63), extended the list of subsidiary titles of the Kings of Sweden considerably. The new title Grand Duke of Finland did not result in any Finnish autonomy, as Finland was an integrated part of the Kingdom of Sweden with full parliamentary representation for its counties. During the next two centuries, the title was used by some of John's successors on the throne, but not all. Usually it was just a subsidiary title of the king, used only on very formal occasions. However, in 1802, as an indication of his resolve to keep Finland within Sweden in the face of increased Russian pressure, King Gustav IV Adolf gave the title to his new-born son, Prince Carl Gustaf, who died three years later.
During the Finnish War between Sweden and Russia, the four Estates of occupied Finland were assembled at the Diet of Porvoo on 29 March 1809 to pledge allegiance to Alexander I of Russia, who in return guaranteed that the area's laws and liberties as well as religion would be left unchanged. Following the Swedish defeat in the war and the signing of the Treaty of Fredrikshamn on 17 September 1809, Finland became a true autonomous grand duchy within the autocratic Russian Empire; but the usual balance of power between monarch and diet resting on taxation was not in place, since the Emperor could rely on the rest of his vast Empire. The title "Grand Duke of Finland" was added to the long list of titles of the Russian Tsar.
After his return to Finland in 1812, the Finnish-born Gustaf Mauritz Armfelt became counsellor to the Russian emperor. Armfelt was instrumental in securing the Grand Duchy as an entity with relatively greater autonomy within the Russian realm, and restoring the so-called Old Finland that had been lost to Russia in the Treaty of Nystad in 1721.
The formation of the Grand Duchy stems from the Treaty of Tilsit between Tsar Alexander I of Russia and Napoleon Bonaparte of France. The treaty mediated peace between Russia and France and allied the two countries against Napoleon's remaining threats: Great Britain and Sweden. Russia invaded Finland in February 1808, claimed as an effort to impose military sanctions against Sweden, but not a war of conquest, and that Russia decided to only temporarily control Finland. Collectively, the Finnish were predominately Anti-Russian, and Finnish guerillas and peasant uprisings were a large obstacles for the Russians, forcing Russia to use various tactics to quash armed Finnish rebellion. Thus, in the beginning of the war, General Friedrich Wilhelm von Buxhoeveden, with permission of the Tsar, issued an oath of fealty on Finland, in which Russia would honor Finland's Lutheran faith, the Finnish Diet, and the Finnish estates as long as the Finns would remain loyal to the Russian crown. The oath also dubbed anyone person who gave aid to the Swedish or Finnish armies a rebel.
The Finns complied, bitter over Sweden abandoning the country for their war against Denmark and France, and begrudgingly embraced Russian conquest. The Diet of Finland was now to only meet whenever requested, and was never mentioned in the manifesto published by the Ministry of Foreign Affairs. Further on, Alexander I requested a deputation of the four Finnish estates, as he expressed concern over continued Finnish resistance. The deputation refused to act without the Diet, to which Alexander agreed with, and promised the Diet would shortly be summoned. By 1809, all of Finland had been conquered and The Diet was summoned in March. Finland was then united through Russia via crown, and Finland was able to keep the majority of its own laws, giving it autonomy.