While 8,000 Swedish and Finnish troops were being deployed at the Russian border, near Lappeenranta (Villmanstrand) and Hamina (Frederikshamn), Sweden declared war on August 8. The objective of these maneuvers was to pose a threat to Saint Petersburg and to set a stage for a coup d'état, engineered by French and Swedish diplomats and aimed at toppling the pro-Austrian regime of Anna Leopoldovna. The coup took place in December, but the new Tsarina, Elizaveta Petrovna, reneged on her promises to return the Baltic provinces to Sweden and continued the vigorous prosecution of the war, under the guidance of her philo-Austrian chancellor, Aleksey Bestuzhev.
The threat to the Russian capital had been relieved as early as September 3, when 20,000 Russians under Field-Marshal Peter Lacy advanced from Vyborg (Viborg, Viipuri) towards Lappeenranta (Villmanstrand) and inflicted a major defeat on the Swedish commander Charles Emil Lewenhaupt. By June 1742, the 35,000-strong Russian army ousted 17,000 Swedes from Hamina (Frederikshamn). As the war unfolded, Lewenhaupt's position deteriorated and he was forced to retreat towards Helsinki (Helsingfors). In August, Lacy captured Porvoo (Borgå) and Savonlinna (Nyslott) and encircled the entire Swedish army near Helsinki (Helsingfors), where he received Lewenhaupt's capitulation on September 4.
As soon as hostilities ceased and the Russian army entered Turku (Åbo), the statesmen Alexander Rumyantsev and Ernst Nolken arrived at the city in order to discuss a peace settlement. The tsarina promised to evacuate her army from Finland on condition that Adolf Frederick of Holstein-Gottorp — that is, the uncle of her own heir apparent — will be named as the heir to the throne of Sweden. (The latter had been offered the crown of Finland by the local lantdag). The Hats acquiesced to her proposal in the hope that Adolf Frederick would be able to obtain better terms from her. Elizabeth also wanted Russian forces to occupy Sweden in order to ensure Adolf Frederick's peaceful election, but this plan aroused vehement opposition of the Swedish representatives and was abandoned.
While peace negotiations lumbered on, the Baltic Fleet of Russia destroyed a Swedish flotilla near Korpo Island (Korppoo). The following month, Lacy — who had distinguished himself in similar operations during the Great Northern War — embarked from Kronstadt in order to effect a landing in Sweden proper. When the Baltic Fleet was approaching Umeå, news came that the Treaty of Åbo (Turku) was finalized, with Sweden ceding to Russia a strip of Finland to the northwest of Saint Petersburg with the towns of Lappeenranta and Hamina. The Kymijoki River was to form part of the border between the two powers. The treaty marked further eclipse of Sweden as a great power in Northern Europe.
The territory ceded to Russia, were joined with the Russian gains in the Treaty of Nystad (Uusikaupunki) in 1721, in the Governorate of Vyborg. They would later be incorporated into the Russian Grand Duchy of Finland in 1812.