The defeat of the Russian army at Friedland in 1807 brought end to the third coalition against France. With this victory and Treaty of Tilsit, Napoleon was virtually the master of western Europe, all the way to new Duchy of Warsaw.
During the negotiations in Tilsit, Napoleon and Alexander I of Russia decided not only on peace. The treaty brought a dramatic change in Europe, not unlike in 1939 Soviet-German Pact, where French and Russia agreed to help each other in disputes and divided Europe into spheres of influence.
They also agreed that Portugal, Denmark and Sweden should become part of continental blockade against England, by force if necessary. And Sweden as a neighbour was for Russia to handle.
Sweden was in very bad strategic position, and had a difficult choice to make. Should it agree and join the continental powers, or continue to resist with the help of the Britain.
The Swedish king, Gustav IV Adolf, decided to protect Swedish maritime trade and committed to oppose Franco-Russian demands. That he also viewed the Napoleon as the Antichrist from the Book of Revelations, made the decision easy for him, but eventually fateful for Sweden.
In the winter of 1808 Russian started the preparations for war and invasion of Finland. During the last months of 1807 the Swedish didn’t make any significant attempts to strengthen their defences, and only in the last minute authorities in Finland ordered the mobilization.
Russian was concentrating two divisions (17th , 21st) for the main thrust against southern Finland. For the secondary front to the north in Savolax, they had one division (5th). Supporting these first units Russia had almost endless reserves.
For the defence of the borders, hastily mobilized Army of Finland had one brigade per Russian division. There where more troops in the rear areas or still on the march from northern provinces, but these included only few battalions and many were second rate units.
The real strength of the Swedish defences in the south was the major fortress of Sveaborg (Suomenlinna) near Helsinki. As long as it was in Swedish hands, Russian could not consolidate any gains in the Southern Finland.
February 21, 1808 Russian troops crossed the border.
Read the whole story of how we recreated key battles of the war on our 15 mm Russo-Swedish war pages.