Sweden, Finland and Russia, with borders and places of importance.
Prodded by Napoleon, Czar Alexander I of Russia invades the Swedish province of Finland with 24 000 men in 3 divisions under command of Lieutenant General Fredrik Vilhelm von Buxhoevden. There has been no declaration of war and the Swedes are taken by surprise. However, the snow still lies deep all over Finland and the Russian advance is slow. The Russian invasion is half-hearted and happens mostly because Gustav IV Adolf of Sweden is the last regent of continental Europe to reject the continental system and Napoleon demands that Alexander do something about it.
As in OTL.
Field Marshal Mauritz Klingspor, on his was to take command of the Swedish forces in Finland, slips and breaks is leg when making quarters in Torneå. There are complications, and a mere three weeks later the gangrene has killed him.
In the absence of the Field Marshal, Lieutenant General Karl Nathanael af Klercker, despite his advanced age (he is 73 years old at this time), reatins command of the Swedish forces in Finland. From his sickbed, Klingspor orders that his pre-war planned retreat must be executed. From diplomatic channels in Stockholm he has intelligence that the Russians have commited at last 60 000 men, somthing the Swedish forces cannot hope to face in open combat before summer, when the ice of the Baltic is gone and the main army can be transported to Finland. In a romantic deed, the General Adjutant Gustaf Löwenhielm stay at Klingspor's side as he fades away from this world.
Thus, there is no-one present in Finland to ensure that Klingspor's orders are followed. Indeed, af Klercker has his own ideas. He agrees that the meagre Swedish forces in Finland cannot face the Russians head on, but in hisplan the retreat is shorter - to Åbo and Tavastehus, rather than Uleåborg as in Klingspor's plan.
In OTL, Klingspor took command and ordered the retreat. Here he is out of the picture.
With small units of light forces, af Klercker harrases and delays the Russian advance. This also provides him with intelligence through recoinnasance that the Russian forces are between 20 000 and 30 000 men strong, not 60 000 as Klingspor assumed. With this knowledge, af Klercker decides to interpret Klingspor's orders bit different than the Field Marshal intended.
The Savolax Brigade, mostly made up of light forces with a core of Finnish Jägers, armed with rifled muskets and under command of Colonel Johan Adam Cronstedt, maintains contact with the Russian forces, harrasing them constantly as it slowly retreats nortward towards Tavastehus, drawing a sizable Russian force under General Michail Bulatov away from the main Russain forces, in pursuit.
The Swedish main force under Lieutenant General Carl Johan Adlercreutz retreats towards Åbo to await reinforcements, while af Klercker himself leads a minor detachment of Jägers (roughly 200 men) to Sveaborg to place himself at the natural arrival point of reinforcements and the starting point of a counter-offensive as soon as the ice has loosened its grip on the Baltic Sea.
The idea is to have three separate forces capable of at least limited operations to distract the Russians until spring.
In Sveaborg, af Klercker is soon surrounded by the Russian main force. However, Sveaborg is one of the largest fortresses of the world even though it is not entirely finished yet. Placed on six islands in the bay just outside Helsingfors, the fortress dominates the sealane and houses 6 200 men equipped with more than 700 guns, althought some of the guns are placed on the 200 ships of the main part of the Swedish archipelago navy that is frozen in the ice at Sveaborg. The Russians lack siege artillery and conduct no immediate attacks, despite the ice giving free access to Sveaborg by foot.
The Fortress of Sveaborg just outside Helsingfors.
In OTL, Klingspor's retreat was carried through and af Klercker's plan rejected. Also, no reinforcements or anyone of higher command were sent to Sveaborg.