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 "Tsar" 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.